vm_page: Remove extra test from page alloc

Extract code from vm_page_alloc_contig_domain into a new function. Do
so in a way that eliminates a bound-to-fail reservation test after a
reservation is broken by a call from vm_page_alloc_contig_domain.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D33551

(cherry picked from commit fb38b29b5609b0c0769b2b2e1d6c917c0799f2a0)