mm/page_ext: Add page_ext_get_from_phys()

mm/page_ext: Add page_ext_get_from_phys()

The IOMMU code operates on physical addresses which can be outside
of system RAM.

Add a new function page_ext_get_from_phys() to abstract the logic of
checking the address and returning the page_ext.

Signed-off-by: Mostafa Saleh <smostafa@google.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>