Prepare for adding psind == 1 support to armv6's pmap_enter().

Precompute the new PTE before entering the critical section.

Eliminate duplication of the pmap and pv list unlock operations in
pmap_enter() by implementing a single return path. Otherwise, the
duplication will only increase with the upcoming support for psind == 1.

Reviewed by: mmel
Tested by: mmel
Discussed with: kib, markj
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D16443