dm: remove EOPNOTSUPP for barriers

If the underlying device doesn't support barriers and dm receives a
barrier, it waits until all requests on that device drain so it no
longer needs to report -EOPNOTSUPP to the caller.

This patch deals with the confusing situation when moving a volume from
one physical device to another triggers an EOPNOTSUPP on a volume that
didn't report it before.
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

drivers/md/dm.c

@@ -555,7 +555,7 @@ static void dec_pending(struct dm_io *io, int error)
 			 * a per-device variable for error reporting.
 			 * Note that you can't touch the bio after end_io_acct
 			 */
-			if (!md->barrier_error)
+			if (!md->barrier_error && io_error != -EOPNOTSUPP)
 				md->barrier_error = io_error;
 			end_io_acct(io);
 		} else {