In particular, we store a FIB number in both struct socket and in struct
inpcb. When updating the FIB number with setsockopt(SO_SETFIB), make
the update atomic. This is required to support the new bind_all_fibs
mode, since in that mode changing the FIB of a bound socket is not
permitted.

This requires a bit more code, but avoids a layering violation in
sosetopt(), where we hard-code the list of protocol families that
implement SO_SETFIB.

Sponsored by: Klara, Inc.
Sponsored by: Stormshield