In preparation for allowing user space to add (*, G) entries with a
source list and associated filter mode, add the necessary plumbing to
handle such requests.

Extend the MDB configuration structure with a currently empty source
array and filter mode that is currently hard coded to EXCLUDE.

Add the source entries and the corresponding (S, G) entries before
making the new (*, G) port group entry visible to the data path.

Handle the creation of each source entry in a similar fashion to how it
is created from the data path in response to received Membership
Reports: Create the source entry, arm the source timer (if needed), add
a corresponding (S, G) forwarding entry and finally mark the source
entry as installed (by user space).

Add the (S, G) entry by populating an MDB configuration structure and
calling br_mdb_add_group_sg() as if a new entry is created by user
space, with the sole difference that the 'src_entry' field is set to
make sure that the group timer of such entries is never armed.

Note that it is not currently possible to add more than 32 source
entries to a port group entry. If this proves to be a problem we can
either increase 'PG_SRC_ENT_LIMIT' or avoid forcing a limit on entries
created by user space.
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>