Eliminate duplicate assertions. We assert at the start of vm_fault_hold()
that the map entry is wired if the caller passes the flag VM_FAULT_WIRE.
Eliminate the same assertion, but spelled differently, at the end of
vm_fault_hold() and vm_fault_populate(). Repeat the assertion only if the
map is unlocked and the map lookup must be repeated.

Reviewed by: kib
MFC after: 10 days
Differential Revision: https://reviews.freebsd.org/D15582