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

[markj]

Summary

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 -1 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.

Sponsored by: Klara, Inc.
Sponsored by: Stormshield

[markj]

Use RT_ALL_FIBS instead of -1 as a sentinel value.