dm thin: fail messages with EOPNOTSUPP when pool cannot handle messages

Use EOPNOTSUPP, rather than EINVAL, error code when user attempts to
send the pool a message.  Otherwise usespace is led to believe the
message failed due to invalid argument.
Reported-by: Zdenek Kabelac <zkabelac@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-thin.c

@@ -3656,7 +3656,7 @@ static int pool_message(struct dm_target *ti, unsigned argc, char **argv)
 	if (get_pool_mode(pool) >= PM_READ_ONLY) {
 		DMERR("%s: unable to service pool target messages in READ_ONLY or FAIL mode",
 		      dm_device_name(pool->pool_md));
-		return -EINVAL;
+		return -EOPNOTSUPP;
 	}
 
 	if (!strcasecmp(argv[0], "create_thin"))