pfsync: replace struct pfsync_pkt with int flags

Get rid of struct pfsync_pkt. It was used to store data on the stack to
pass to all the submessage handlers, but only the flags part of it was
ever used. Just pass the flags directly instead.

Reviewed by: kp
Obtained from: OpenBSD
Sponsored by: InnoGames GmbH
Differential Revision: https://reviews.freebsd.org/D36294