Fix assert condition in pmap_large_unmap().

pmap_large_unmap() asserts that an unmapping request covers the
entirety of a 2M or 1G page. The logic in the asserts was out of date
with the loop logic. Correct the test to actually check that
destroying the current superpage mapping does not unmap addresses
beyond those requested by the caller.

Submitted by: D Scott Phillips <d.scott.phillips@intel.com>
Reviewed by: alc
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D18345