Avoid needless TLB invalidations in pmap_remove_pages().

pmap_remove_pages() is called during process termination, when it is
guaranteed that no other CPU may access the mappings being torn down.
In particular, it unnecessary to invalidate each mapping individually
since we do a pmap_invalidate_all() at the end of the function.

Also don't call pmap_invalidate_all() while holding a PV list lock, the
global pvh lock is sufficient.

Reviewed by: jhb
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D18562