Add a real pfn_valid

This adds a real pfn_valid.  Needed together with the SRAT patch which can
generated memmap less holes for mmio.  In general it will help to handle
memory holes better.
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>