inpcb: Add FIB-aware inpcb lookup

Allow protocol layers to look up an inpcb belonging to a particular FIB.
This is indicated by setting INPLOOKUP_FIB; if it is set, the FIB to be
used is obtained from the specificed mbuf or ifnet.

No functional change intended.

Reviewed by: glebius, melifaro
MFC after: 2 weeks
Sponsored by: Klara, Inc.
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D48662

(cherry picked from commit da806e8db685eead02bc67888b16ebac6badb6b6)