pf: Fix definitions of pf_pfil_*_hooked

This use of "volatile" in the vnet definitions doesn't have any effect.
VNET_DEFINE_STATE(volatile int, ...) should work, but let's avoid using
"volatile" altogether and convert to atomic_load/atomic_store. Also
convert to bool while here.

Reviewed by: kp, mjg
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37684