powerpc: Fix pfn_valid() when memory starts at a non-zero address

max_mapnr is a pfn, not an index innto mem_map[].  So don't add
ARCH_PFN_OFFSET a second time.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>