inpcb: Imbue in(6)_pcblookup_local() with a FIB parameter

This is to enable a mode where duplicate inpcb bindings are permitted,
and we want to look up an inpcb with a particular FIB. Thus, add a
"fib" parameter to in_pcblookup() and related functions, and plumb it
through.

A fib value of RT_ALL_FIBS indicates that the lookup should ignore FIB
numbers when searching. Otherwise, it should refer to a valid FIB
number, and the returned inpcb should belong to the specific FIB. For
now, just add the fib parameter where needed, as there are several
layers to plumb through.

No functional change intended.

Reviewed by: glebius
MFC after: 2 weeks
Sponsored by: Klara, Inc.
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D48660