vm_fault: Stop specifying VM_ALLOC_ZERO

Now vm_page_alloc() and friends will unconditionally preserve PG_ZERO,
so there is no point in setting this flag.

Eliminate a local variable and add a comment explaining why we
prioritize the allocation when the process is doomed.

No functional change intended.

Reviewed by: kib, alc
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D32036