MFC r313820

MFC r313820

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.

Relnotes:	yes (if I/someone can test pf OS fingerprinting with IPv6)
Sponsored by:	Dell EMC

git-svn-id: svn://svn.freebsd.org/base/stable/10@315028 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f