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
    
    (cherry picked from commit 9a4131629bb3083ddc02a32950e4eb4806a07710)