BUG/MINOR: log: set proper smp size for balance log-hash

BUG/MINOR: log: set proper smp size for balance log-hash

result.data.u.str.size was set to size+1 to take into account terminating
NULL byte as per the comment. But this is wrong because the caller is free
to set size to just the right amount of bytes (without terminating NULL
byte). In fact all smp API functions will not read past str.data so there
is not risk about uninitialized reads, but this leaves an ambiguity for
converters that may use all the smp size to perform transformations, and
since we don't know about the "message" memory origin, we cannot assume
that its size may be greater than size. So we max it out to size just to
be safe.

This bug was not known to cause any issue, it was spotted during code
review. It should be backported in 2.9 with b30bd7a ("MEDIUM: log/balance:
support for the "hash" lb algorithm")