Fix too loose assert in pmap_large_unmap().

The upper bound for the valid address from the large map used
LARGEMAP_MAX_ADDRESS instead of LARGEMAP_MIN_ADDRESS. Provide a
function-like macro for proper upper value.

Noted by: markj
Reviewed by: alc, markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D20386