bridge: do not call setup_timer() multiple times

commit 9f00b2e7cf24 ("bridge: only expire the mdb entry when query is
received") added a nasty bug as an active timer can be reinitialized.

setup_timer() must be done once, no matter how many time mod_timer()
is called. br_multicast_new_group() is the right place to do this.

Reported-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Diagnosed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Tested-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Cc: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>