network: replace detailed netlink append messages with a single generic message

network: replace detailed netlink append messages with a single generic message

This commit is the first in the series, and they generally follow the same
idea: we had very detailed logging for message append operations which would
only fail either with some type error or intrinsic limit (and then they would
fail everywhere, so this would be noticed during development or in CI), or they
would fail with ENOMEM, in which case the exact location is not very interesting
since this is not repeatable.

I am in general in favour of detailed logging messages, because it helps with
diagnosis of errors, but I think case is an exception. Despite not being very
useful, those messages required a lot of effort, because they were customized
for each and every append operation. In fact some of the messages contained copy
errors. The text of the messages (since they are generally unique) also added up
to a considerable size.

This removes the log messages after each sd_netlink_message_append_*() in
fill_message_create() with a single line in netdev_create(). As described
above, we are just appending fields to a message, so those calls would almost
never fail.

A forgotten 'return' was added in one place.

$ size build/systemd-networkd{.0,}
   text	   data	    bss	    dec	    hex	filename
1878634	 394016	     36	2272686	 22adae	build/systemd-networkd.0
1842450	 394080	     36	2236566	 222096	build/systemd-networkd

… so we save 30k too.