vm_page: Remove extra test, dup code from page alloc

Extract code common to functions vm_page_alloc_contig_domain and
vm_page_alloc_noobj_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