x86: Change get_max_mapped() to inline

Move it into head file. to prepare use it in other files.

[ hpa: added missing <linux/types.h> and changed type to phys_addr_t. ]

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
LKML-Reference: <4D1933BA.8000508@kernel.org>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>