net: dsa: call dsa_port_bridge_join when joining a LAG that is already in a bridge

DSA can properly detect and offload this sequence of operations:

ip link add br0 type bridge
ip link add bond0 type bond
ip link set swp0 master bond0
ip link set bond0 master br0

But not this one:

ip link add br0 type bridge
ip link add bond0 type bond
ip link set bond0 master br0
ip link set swp0 master bond0

Actually the second one is more complicated, due to the elapsed time
between the enslavement of bond0 and the offloading of it via swp0, a
lot of things could have happened to the bond0 bridge port in terms of
switchdev objects (host MDBs, VLANs, altered STP state etc). So this is
a bit of a can of worms, and making sure that the DSA port's state is in
sync with this already existing bridge port is handled in the next
patches.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Tobias Waldekranz <tobias@waldekranz.com>
Signed-off-by: David S. Miller <davem@davemloft.net>