Drain for the xbusy state for two places which potentially do
pmap_remove_all(). Not doing the drain allows the pmap_enter() to
proceed in parallel, making the pmap_remove_all() effects void.

The race results in an invalidated page mapped wired by usermode.

Reported and tested by:	pho
Reviewed by:	alc
Sponsored by:	The FreeBSD Foundation
Approved by:	re (glebius)
Relese the interface in the last.

Reviewed by:	glebius
Approved by:	re (kib)