bridge: mcast: Factor out a helper for PG entry size calculation

Currently, netlink notifications are sent for individual port group
entries and not for the entire MDB entry itself.

Subsequent patches are going to add MDB get support which will require
the bridge driver to reply with an entire MDB entry.

Therefore, as a preparation, factor out an helper to calculate the size
of an individual port group entry. When determining the size of the
reply this helper will be invoked for each port group entry in the MDB
entry.

No functional changes intended.

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