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: 0436862e ("net: bridge: mcast: support for IGMPv3/MLDv2 ALLOW_NEW_SOURCES report")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>