bridge: mcast: Avoid arming group timer when (S, G) corresponds to a source

User space will soon be able to install a (*, G) with a source list,
prompting the creation of a (S, G) entry for each source.

In this case, the group timer of the (S, G) entry should never be set.

Solve this by adding a new field to the MDB configuration structure that
denotes whether the (S, G) corresponds to a source or not.

The field will be set in a subsequent patch where br_mdb_add_group_sg()
is called in order to create a (S, G) entry for each user provided
source.

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