Add a helper function for validating VA ranges.

Functions which take untrusted user ranges must validate against the
bounds of the map, and also check for wraparound. Instead of having the
same logic duplicated in a number of places, add a function to check.

Reviewed by: dougm, kib
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D25328