Fix garbage IP addresses in UDP log_in_vain messages

If multiple threads emit a UDP log_in_vain message concurrently,
the IP addresses could be garbage due to concurrent usage of a
single string buffer inside inet_ntoa(). Use inet_ntoa_r() with
two stack buffers instead.

Reported by: Mark Martinec <Mark.Martinec+freebsd@ijs.si>
MFC after: 3 days
Relnotes: yes
Sponsored by: Dell EMC