net: bridge: multicast: rename src_size to addr_size

Rename src_size argument to addr_size in preparation for passing host
address as an argument to IGMPv3/MLDv2 functions.
No functional change.
Signed-off-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>