[GENETLINK]: Correctly report errors while registering a multicast group

Signed-off-by: NThomas Graf <tgraf@suug.ch>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/netlink/genetlink.c

@@ -196,7 +196,7 @@ int genl_register_mc_group(struct genl_family *family,
 	genl_ctrl_event(CTRL_CMD_NEWMCAST_GRP, grp);
  out:
 	genl_unlock();
-	return 0;
+	return err;
 }
 EXPORT_SYMBOL(genl_register_mc_group);