x86, mm: use pfn instead of pos in split_mem_range

could save some bit shifting operations.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Link: http://lkml.kernel.org/r/1353123563-3103-38-git-send-email-yinghai@kernel.org


Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>