Assert the VM map lock is held in function used by mmap/mprotect/munmap.

Assert the VM map lock is held in function used by mmap/mprotect/munmap.

Also grab the lock in uvm_map_teardown() and uvm_map_deallocate() to
satisfy the assertions.  Grabbing the lock there shouldn't be strictly
necessary, because no other reference to the map should exist when the
reaper is holding it, but it doesn't hurt and makes our life easier.

Inputs & tests from Ivo van der Sangen, tb@, gnezdo@, kn@

kettenis@ and tb@ agree with the direction, ok kn@