x86, mm: Use new pagetable helpers in try_preserve_large_page()

try_preserve_large_page() can be slightly simplified by using
the new page_level_*() helpers.  This also moves the 'level'
over to the new pg_level enum type.

Signed-off-by: Dave Hansen <dave@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/20130122212432.14F3D993@kernel.stglabs.ibm.com


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