Avoid overflow possibility.

[ The overflow is purely theoretical, since this is used for memory
  ranges that aren't even close to using the full 64 bits, but this is
  the right thing to do regardless.  - Linus ]

Signed-off-by: Louis Langholtz <lou_langholtz@me.com>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: Peter Anvin <hpa@linux.intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>