dm snapshot: don't define direct_access if we don't support it

Don't define a direct_access function that fails, dm_dax_direct_access
already fails with -EIO if the pointer is zero;
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-snap.c

@@ -2338,13 +2338,6 @@ static int origin_map(struct dm_target *ti, struct bio *bio)
 	return do_origin(o->dev, bio);
 }
 
-static long origin_dax_direct_access(struct dm_target *ti, pgoff_t pgoff,
-		long nr_pages, void **kaddr, pfn_t *pfn)
-{
-	DMWARN("device does not support dax.");
-	return -EIO;
-}
-
 /*
  * Set the target "max_io_len" field to the minimum of all the snapshots'
  * chunk sizes.
@@ -2404,7 +2397,6 @@ static struct target_type origin_target = {
 	.postsuspend = origin_postsuspend,
 	.status  = origin_status,
 	.iterate_devices = origin_iterate_devices,
-	.direct_access = origin_dax_direct_access,
 };
 
 static struct target_type snapshot_target = {