journal: treble field hash table size

journal: treble field hash table size

As discussed in https://github.com/systemd/systemd/issues/38399, "ordinary"
systems can have the field table with a large number of values, causing journal
rotation to occur early. For example, audit generates a log of fields:
$ journalctl --fields | rg -c '^_?AUDIT'
114

It seems that the "structured log" capabilities of the journal are being use
more than in the past. Looking at some journal files on my system, it seems
the field hash table field is quite high in many cases:
$ build/test-journal-dump /var/log/journal/*/* | rg 'table fill'
Data hash table fill: 15.1%
Field hash table fill: 69.1%
Data hash table fill: 4.9%
Field hash table fill: 32.4%
Data hash table fill: 10.2%
Field hash table fill: 34.2%
Data hash table fill: 9.9%
Field hash table fill: 37.2%
Data hash table fill: 26.8%
Field hash table fill: 21.9%
Data hash table fill: 35.6%
Field hash table fill: 22.8%
Data hash table fill: 25.5%
Field hash table fill: 54.1%
Data hash table fill: 3.4%
Field hash table fill: 43.8%
Data hash table fill: 75.0%
Field hash table fill: 70.3%
Data hash table fill: 75.0%
Field hash table fill: 63.1%
Data hash table fill: 75.0%
Field hash table fill: 74.2%
Data hash table fill: 35.6%
Field hash table fill: 43.2%
Data hash table fill: 35.5%
Field hash table fill: 75.4%
Data hash table fill: 75.0%
Field hash table fill: 59.8%
Data hash table fill: 75.0%
Field hash table fill: 56.5%
Data hash table fill: 16.9%
Field hash table fill: 76.3%
Data hash table fill: 18.1%
Field hash table fill: 76.9%
Data hash table fill: 75.0%
Field hash table fill: 42.0%
Data hash table fill: 75.0%
Field hash table fill: 22.8%
Data hash table fill: 75.0%
Field hash table fill: 22.8%
Data hash table fill: 75.0%
Field hash table fill: 22.8%
Data hash table fill: 75.0%
Field hash table fill: 22.8%
Data hash table fill: 75.0%
Field hash table fill: 32.1%
Data hash table fill: 75.0%
Field hash table fill: 21.9%
Data hash table fill: 75.0%
Field hash table fill: 21.9%
Data hash table fill: 75.0%
Field hash table fill: 21.9%
Data hash table fill: 75.0%
Field hash table fill: 22.8%
Data hash table fill: 75.0%
Field hash table fill: 22.8%
Data hash table fill: 75.0%
Field hash table fill: 21.9%
Data hash table fill: 75.0%
Field hash table fill: 22.5%
Data hash table fill: 9.6%
Field hash table fill: 53.8%
Data hash table fill: 75.0%
Field hash table fill: 22.2%
Data hash table fill: 75.0%
Field hash table fill: 22.2%
Data hash table fill: 75.0%
Field hash table fill: 22.2%
Data hash table fill: 35.6%
Field hash table fill: 75.1%
Data hash table fill: 33.6%
Field hash table fill: 50.2%
Data hash table fill: 75.0%
Field hash table fill: 26.7%
Data hash table fill: 75.0%
Field hash table fill: 25.8%
Data hash table fill: 75.0%
Field hash table fill: 29.1%
Data hash table fill: 75.0%
Field hash table fill: 25.8%
Data hash table fill: 75.0%
Field hash table fill: 31.8%
Data hash table fill: 75.0%
Field hash table fill: 18.9%
Data hash table fill: 75.0%
Field hash table fill: 22.2%
Data hash table fill: 75.0%
Field hash table fill: 20.1%
Data hash table fill: 75.0%
Field hash table fill: 29.1%
Data hash table fill: 75.0%
Field hash table fill: 30.9%
Data hash table fill: 75.0%
Field hash table fill: 28.5%
Data hash table fill: 75.0%
Field hash table fill: 28.5%
Data hash table fill: 75.0%
Field hash table fill: 25.8%
Data hash table fill: 75.0%
Field hash table fill: 25.2%
Data hash table fill: 75.0%
Field hash table fill: 39.3%
Data hash table fill: 50.2%
Field hash table fill: 75.1%
Data hash table fill: 75.0%
Field hash table fill: 61.9%
Data hash table fill: 75.0%
Field hash table fill: 56.5%
Data hash table fill: 75.0%
Field hash table fill: 58.6%
Data hash table fill: 48.9%
Field hash table fill: 79.6%
Data hash table fill: 75.0%
Field hash table fill: 71.5%
Data hash table fill: 75.0%
Field hash table fill: 60.1%
Data hash table fill: 31.4%
Field hash table fill: 75.7%
Data hash table fill: 27.0%
Field hash table fill: 69.4%
Data hash table fill: 28.9%
Field hash table fill: 76.6%
Data hash table fill: 60.2%
Field hash table fill: 79.9%
Data hash table fill: 8.8%
Field hash table fill: 78.7%
Data hash table fill: 5.8%
Field hash table fill: 61.3%
Data hash table fill: 75.0%
Field hash table fill: 64.0%
Data hash table fill: 61.4%
Field hash table fill: 63.4%
Data hash table fill: 29.7%
Field hash table fill: 61.9%
Data hash table fill: 18.9%
Field hash table fill: 30.9%
Data hash table fill: 1.4%
Field hash table fill: 22.2%
Data hash table fill: 0.4%
Field hash table fill: 13.5%
Data hash table fill: 2.6%
Field hash table fill: 37.5%
Data hash table fill: 1.3%
Field hash table fill: 23.4%
Data hash table fill: 0.6%
Field hash table fill: 15.3%
Data hash table fill: 18.7%
Field hash table fill: 33.9%
Data hash table fill: 7.4%
Field hash table fill: 37.5%
Data hash table fill: 20.2%
Field hash table fill: 44.1%
Data hash table fill: 1.3%
Field hash table fill: 33.0%
Data hash table fill: 75.0%
Field hash table fill: 19.2%
Data hash table fill: 42.2%
Field hash table fill: 23.4%
Data hash table fill: 1.6%
Field hash table fill: 87.1%
Data hash table fill: 0.1%
Field hash table fill: 98.8%
Data hash table fill: 0.2%
Field hash table fill: 128.8%
Data hash table fill: 15.4%
Field hash table fill: 31.2%
Data hash table fill: 7.4%
Field hash table fill: 22.5%
Data hash table fill: 10.5%
Field hash table fill: 38.7%
Data hash table fill: 2.8%
Field hash table fill: 18.0%
Data hash table fill: 1.5%
Field hash table fill: 15.9%
Data hash table fill: 0.0%
Field hash table fill: 7.5%
Data hash table fill: 0.1%
Field hash table fill: 12.0%
Data hash table fill: 0.2%
Field hash table fill: 10.8%
Data hash table fill: 0.2%
Field hash table fill: 15.6%
Data hash table fill: 0.1%
Field hash table fill: 11.7%
Data hash table fill: 0.1%
Field hash table fill: 12.0%
Data hash table fill: 0.0%
Field hash table fill: 6.6%
Data hash table fill: 1.4%
Field hash table fill: 18.0%
Data hash table fill: 0.7%
Field hash table fill: 16.8%
Data hash table fill: 1.1%
Field hash table fill: 18.0%
Data hash table fill: 0.2%
Field hash table fill: 10.8%
Data hash table fill: 0.1%
Field hash table fill: 10.8%
Data hash table fill: 0.4%
Field hash table fill: 11.1%

Since filling of the field hash table to 75% normally causes file rotation,
let's double the default to make rotation happen less often.
We'll use 11kB more for the hash table, which should be fine, considering
that journal files are usually at least 8 MB.

Closes https://github.com/systemd/systemd/issues/38399.