vm_phys: alloc pages without duplicating searches.

In the search for contiguous pages, as each page segment is examined,
check to see if the free list set for the next page segment differs
from the set for the current segment, and avoid a pointless search if
they do not differ.

Discussed with: alc
Reviewed by: markj
Tested by: pho
Differential Revision: https://reviews.freebsd.org/D33947