We can't use MAP_ENTRY_IN_TRANSITION / vm_map_unlock_and_wait when we
have a vnode lock held, because it can deadlock as vm_map_wire tries to
fault it in.

Bug discovered by Peter Holm (mmap7 / mmap7a from stress2).

Patch suggested by Attilio Rao.

Run mmap7a test; before this patch, the machine eventually deadlocks.
Characteristically, one thread will be waiting on "vmmaps" and at least one
other thread on "ufs".

After this patch, mmap7a no longer causes this deadlock.