Do not try to unmark MAP_ENTRY_IN_TRANSITION marked by other thread.

The issue is catched by "vm_map_wire: alien wire" KASSERT at the end
of the vm_map_wire(). We currently check for MAP_ENTRY_WIRE_SKIPPED
flag before ensuring that the wiring_thread is curthread. For HOLESOK
wiring, this means that we might see WIRE_SKIPPED entry from different
wiring.

The fix it by only checking WIRE_SKIPPED if the entry is put
IN_TRANSITION by us. Also fixed a typo in the comment explaining the
situation.

Reported and tested by: pho
Reviewed by: alc
Sponsored by: The FreeBSD Foundation
MFC after: 1 week