vm_fault: assert that first_m is xbusy

There are several invariants, most important of which is prevention of
parallel faults handling, that depend on first_m being xbusy for the
duration of the page fault processing.

Suggested by: markj
Reviewed by: alc, markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D51474