net: bridge: mcast: Use ERR_CAST instead of ERR_PTR(PTR_ERR())

Use ERR_CAST inlined function instead of ERR_PTR(PTR_ERR(...)).

net/bridge/br_multicast.c:1246:9-16: WARNING: ERR_CAST can be used with mp
Generated by: scripts/coccinelle/api/err_cast.cocci

Signed-off-by: Xu Wang <vulab@iscas.ac.cn>
Link: https://lore.kernel.org/r/20210204070549.83636-1-vulab@iscas.ac.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/bridge/br_multicast.c

@@ -1251,7 +1251,7 @@ __br_multicast_add_group(struct net_bridge *br,
 
 	mp = br_multicast_new_group(br, group);
 	if (IS_ERR(mp))
-		return ERR_PTR(PTR_ERR(mp));
+		return ERR_CAST(mp);
 
 	if (!port) {
 		br_multicast_host_join(mp, true);