bridge: Fix potential deadlock on br->multicast_lock

multicast_lock is taken in softirq context, so we should use
spin_lock_bh() in userspace.

call-chain in softirq context:
run_timer_softirq()
	br_multicast_query_expired()

call-chain in userspace:
sysfs_write_file()
	store_multicast_snooping()
		br_multicast_toggle()

Signed-off-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>