If sysUnused is passed an address or length that is not aligned to the
physical page boundary, the kernel will unmap more memory than the
caller wanted. Add a check for this.

For #9993.

Change-Id: I68ff03032e7b65cf0a853fe706ce21dc7f2aaaf8
Reviewed-on: https://go-review.googlesource.com/22065Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Michael Hudson-Doyle <michael.hudson@canonical.com>