Define vm_map_entry_in_transition to handle an in-transition map
entry, combining code currently in vm_map_unwire and
vm_map_wire_locked into a single function, called by each of them for
entries in transition.

Discussed with: kib, markj
Reviewed by: alc
Approved by: kib, markj (mentors, implicit)
Tested by: pho
Differential Revision: https://reviews.freebsd.org/D20833