arm64: a few simplications to pmap_remove_{all,write}

Eliminate some unnecessary unlocking and relocking when we have to retry
the operation to avoid deadlock. (All of the other pmap functions that
iterate over a PV list already implemented retries without these same
unlocking and relocking operations.)

Avoid a pointer dereference by using an existing local variable that
already holds the desired value.

Eliminate some unnecessary repetition of code on a failed fcmpset.
Specifically, there is no point in retesting the DBM bit because it
cannot change state while the pmap lock is held.

Reviewed by: kib, markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D30931