dsa: Allow forwarding of redirected IGMP traffic

The driver for Marvell switches puts all ports in IGMP snooping mode
which results in all IGMP/MLD frames that ingress on the ports to be
forwarded to the CPU only.

The bridge code in the kernel can then interpret these frames and act
upon them, for instance by updating the mdb in the switch to reflect
multicast memberships of stations connected to the ports. However,
the IGMP/MLD frames must then also be forwarded to other ports of the
bridge so external IGMP queriers can track membership reports, and
external multicast clients can receive query reports from foreign IGMP
queriers.

Currently, this is impossible as the EDSA tagger sets offload_fwd_mark
on the skb when it unwraps the tagged frames, and that will make the
switchdev layer prevent the skb from egressing on any other port of
the same switch.

To fix that, look at the To_CPU code in the DSA header and make
forwarding of the frame possible for trapped IGMP packets.

Introduce some #defines for the frame types to make the code a bit more
comprehensive.

This was tested on a Marvell 88E6352 variant.

Signed-off-by: Daniel Mack <daniel@zonque.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>