arm64: Simplify fcmpset failure in pmap_promote_l2()

When the initial fcmpset in pmap_promote_l2() fails, there is no need
to repeat the check for the physical address being 2MB aligned or for
the accessed bit being set. While the pmap is locked the hardware can
only transition the accessed bit from 0 to 1, and we have already
determined that it is 1 when the fcmpset fails.

(cherry picked from commit 0add3c9945c85c7f766f9225866e99e2a805819b)