net: bridge: mcast: fix unused br var when lockdep isn't defined

Stephen reported the following warning:
net/bridge/br_multicast.c: In function 'br_multicast_find_port':
net/bridge/br_multicast.c:1818:21: warning: unused variable 'br' [-Wunused-variable]
1818 | struct net_bridge *br = mp->br;
| ^~

It happens due to bridge's mlock_dereference() when lockdep isn't defined.
Silence the warning by annotating the variable as __maybe_unused.

Fixes: 0436862e417e ("net: bridge: mcast: support for IGMPv3/MLDv2 ALLOW_NEW_SOURCES report")
Reported-by: Stephen Rothwell <[e-mail]>
Signed-off-by: Nikolay Aleksandrov <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/bridge/br_multicast.c

···
 		       struct net_bridge_port *p,
 		       const unsigned char *src)
 {
+	struct net_bridge *br __maybe_unused = mp->br;
 	struct net_bridge_port_group *pg;
-	struct net_bridge *br = mp->br;
 
 	for (pg = mlock_dereference(mp->ports, br);
 	     pg;