pfsync: handle large MTU pfsync interfaces

pfsync packets were allocated with m_get2(), which can't return packets
larger than MJUMPAGESIZE. As a result 9k MTU pfsync interfaces simply didn't work.

Use m_get3(), which can allocate sufficiently large mbufs.

Extend the pfsync:bulk test case to provoke this problem.

PR: 297307
MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit 7e2781fdcfdbe489cc07572d33dc36bca06a342d)