Move the new ipf_pcksum6() function from ip_fil_freebsd.c to fil.c.

Move the new ipf_pcksum6() function from ip_fil_freebsd.c to fil.c.
The reason for this is that ipftest(8), which still works on FreeBSD-11,
fails to link to it, breaking stable/11 builds.

ipftest(8) was broken (segfault) sometime during the FreeBSD-12 cycle.
glebius@ suggested we disable building it until I can get around to
fixing it. Hence this was not caught in -current.

The intention is to fix ipftest(8) as it is used by the netbsd-tests
(imported by ngie@ many moons ago) for regression testing.

MFC after:	immediately