userfaultfd: shmem: wire up shmem_mfill_zeropage_pte

For shmem VMAs we can use shmem_mfill_zeropage_pte for UFFDIO_ZEROPAGE

Link: http://lkml.kernel.org/r/1497939652-16528-6-git-send-email-rppt@linux.vnet.ibm.com
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/userfaultfd.c

@@ -389,11 +389,13 @@ static __always_inline ssize_t mfill_atomic_pte(struct mm_struct *dst_mm,
 			err = mfill_zeropage_pte(dst_mm, dst_pmd,
 						 dst_vma, dst_addr);
 	} else {
-		err = -EINVAL; /* if zeropage is true return -EINVAL */
-		if (likely(!zeropage))
+		if (!zeropage)
 			err = shmem_mcopy_atomic_pte(dst_mm, dst_pmd,
 						     dst_vma, dst_addr,
 						     src_addr, page);
+		else
+			err = shmem_mfill_zeropage_pte(dst_mm, dst_pmd,
+						       dst_vma, dst_addr);
 	}
 
 	return err;