Add a flag, INPBIND_FIB, which means that the inpcb is local to its FIB
number. When this flag is specified, duplicate bindings are permitted,
so long as each FIB contains at most one inpcb bound to the same
address/port. If an inpcb is bound with this flag, it'll have the
INP_BOUNDFIB flag set.
No functional change intended.
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