pf: use inet_ntoa_r() instead of inet_ntoa(); maybe fix IPv6 OS fingerprinting

pf: use inet_ntoa_r() instead of inet_ntoa(); maybe fix IPv6 OS fingerprinting

inet_ntoa() cannot be used safely in a multithreaded environment
because it uses a static local buffer. Instead, use inet_ntoa_r()
with a buffer on the caller's stack.

This code had an INET6 conditional before this commit, but opt_inet6.h
was not included, so INET6 was never defined.  Apparently, pf's OS
fingerprinting hasn't worked with IPv6 for quite some time.
This commit might fix it, but I didn't test that.

Reviewed by:	gnn, kp
MFC after:	2 weeks
Relnotes:	yes (if I/someone can test pf OS fingerprinting with IPv6)
Sponsored by:	Dell EMC
Differential Revision:	https://reviews.freebsd.org/D9625