net: bridge: multicast: use non-bh rcu flavor

The bridge multicast code has been using a mix of RCU and RCU-bh flavors
sometimes in questionable way. Since we've moved to rhashtable just use
non-bh RCU everywhere. In addition this simplifies freeing of objects
and allows us to remove some unnecessary callback functions.

v3: new patch

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>