from enc_hhook().
This should solve the problem when pf is used with if_enc(4) interface,
and outbound packet with existing PCB checked by pf, and this leads to
deadlock due to pf does its own PCB lookup and tries to take rlock when
wlock is already held.
Now we pass PCB pointer if it is known to the pfil hook, this helps to
avoid extra PCB lookup and thus rlock acquiring is not needed.
For inbound packets it is safe to pass NULL, because we do not held any
PCB locks yet.
PR: 220217
MFC after: 3 weeks
Sponsored by: Yandex LLC
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