mm/follow_page_mask: add support for hugetlb pgd entries

mm/follow_page_mask: add support for hugetlb pgd entries

ppc64 supports pgd hugetlb entries.  Add code to handle hugetlb pgd
entries to follow_page_mask so that ppc64 can switch to it to handle
hugetlbe entries.

Link: http://lkml.kernel.org/r/1494926612-23928-5-git-send-email-aneesh.kumar@linux.vnet.ibm.com
Signed-off-by: Anshuman Khandual <[email protected]>
Signed-off-by: Aneesh Kumar K.V <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Benjamin Herrenschmidt <[email protected]>
Cc: Mike Kravetz <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>