amd64 pmap: do not sleep in pmap_allocpte_alloc() with zero referenced page table page.

Otherwise parallel pmap_allocpte_alloc() for nearby va might also fail
allocating page table page and free the page under us. The end result is
that we could dereference unmapped pte when doing cleanup after sleep.

Instead, on allocation failure, first free everything, only then we can
drop pmap mutex and sleep safely, right before returning to caller.
Split inner non-sleepable part of the pmap_allocpte_alloc() into a new
helper pmap_allocpte_nosleep().

Reviewed by: markj
Reported and tested by: pho
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D27956