vm: Don't break vm reserv that can't meet align reqs

Function vm_reserv_test_contig has incorrectly used its alignment
and boundary parameters to find a well-positioned range of empty pages
in a reservation. Consequently, a reservation could be broken
mistakenly when it was unable to provide a satisfactory set of pages.

Rename the function, correct the errors, and add assertions to detect
the error in case it appears again.

Reviewed by: alc, markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D33344