convert pmap_lock into an IPL_VM lock, and avoid calling
KASSERT(!mutex_owned(&pmap_lock)) since that's no longer valid.

this fixes hangs on 4-core machines like the U80.

tested on U80, SB2500 and SB2000.

XXX: my initial attempts to replace pseg_lock with this new equivalent
XXX: lock hang in pmap_bootstrap it seems..  but this should be able
XXX: to go away now.

(mrg)