pmap: Skip some superpage promotion attempts that will fail

Implement a simple heuristic to skip pointless promotion attempts by
pmap_enter_quick_locked() and moea64_enter(). Specifically, when
vm_fault() calls pmap_enter_quick() to map neighboring pages at the end
of a copy-on-write fault, there is no point in attempting promotion in
pmap_enter_quick_locked() and moea64_enter(). Promotion will fail
because the base pages have differing protection.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D45431
MFC after: 1 week