bridge: mcast: Use spin_lock() instead of spin_lock_bh()

IGMPv3 / MLDv2 Membership Reports are only processed from the data path
with softIRQ disabled, so there is no need to call spin_lock_bh(). Use
spin_lock() instead.

This is consistent with how other IGMP / MLD packets are processed.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>