bridge: mcast: Add a centralized error path

Subsequent patches will add memory allocations in br_mdb_config_init()
as the MDB configuration structure will include a linked list of source
entries. This memory will need to be freed regardless if br_mdb_add()
succeeded or failed.

As a preparation for this change, add a centralized error path where the
memory will be freed.

Note that br_mdb_del() already has one error path and therefore does not
require any changes.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>