net: bridge: mst: Add helper to map an MSTI to a VID set

br_mst_get_info answers the question: "On this bridge, which VIDs are
mapped to the given MSTI?"

This is useful in switchdev drivers, which might have to fan-out
operations, relating to an MSTI, per VLAN.

An example: When a port's MST state changes from forwarding to
blocking, a driver may choose to flush the dynamic FDB entries on that
port to get faster reconvergence of the network, but this should only
be done in the VLANs that are managed by the MSTI in question.

Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>