userfaultfd: hugetlbfs: gup: support VM_FAULT_RETRY

userfaultfd: hugetlbfs: gup: support VM_FAULT_RETRY

Orabug: 21685254

Add support for VM_FAULT_RETRY to follow_hugetlb_page() so that
get_user_pages_unlocked/locked and "nonblocking/FOLL_NOWAIT" features will
work on hugetlbfs.  This is required for fully functional userfaultfd
non-present support on hugetlbfs.

Link: http://lkml.kernel.org/r/20161216144821.5183-25-aarcange@redhat.com
Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: Michael Rapoport <RAPOPORT@il.ibm.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
(cherry picked from linux-next next-20170117
 commit b33127bd2a0367d093bfeb1abd147754ff90e670)
Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: Dhaval Giani <dhaval.giani@oracle.com>
Signed-off-by: Dhaval Giani <dhaval.giani@oracle.com>
 Conflicts:
	mm/hugetlb.c