bridge: mcast: Split (*, G) and (S, G) addition into different functions

When the bridge is using IGMP version 3 or MLD version 2, it handles the
addition of (*, G) and (S, G) entries differently.

When a new (S, G) port group entry is added, all the (*, G) EXCLUDE
ports need to be added to the port group of the new entry. Similarly,
when a new (*, G) EXCLUDE port group entry is added, the port needs to
be added to the port group of all the matching (S, G) entries.

Subsequent patches will create more differences between both entry
types. Namely, filter mode and source list can only be specified for (*,
G) entries.

Given the current and future differences between both entry types,
handle the addition of each entry type in a different function, thereby
avoiding the creation of one complex function.

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