Extract the innermost loop of pmap_remove() out into its own function,
pmap_remove_ptes(). (This new function will also be used by an upcoming
change to pmap_enter() that adds support for psind == 1 mappings.)

Submitted by: Yufeng Zhou <yz70@rice.edu> (an earlier version)
Reviewed by: kib, markj
MFC after: 1 week