On a failed fcmpset don't pointlessly repeat tests

In a few places, on a failed compare-and-set, both the amd64 pmap and
the arm64 pmap repeat tests on bits that won't change state while the
pmap is locked. Eliminate some of these unnecessary tests.

Reviewed by: andrew, kib, markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31014