mlxsw: spectrum: Trap loop-backed packets

One of the conditions to generate an ICMP Redirect Message is that "the
packet is being forwarded out the same physical interface that it was
received from" (RFC 1812).

Therefore, we need to be able to trap such packets and let the kernel
decide what to do with them.

For each RIF, enable the loop-back filter, which will raise the LBERROR
trap whenever the ingress RIF equals the egress RIF.

Fixes: 99724c18

 ("mlxsw: spectrum: Introduce support for router interfaces")
Reported-by: Ilan Tayari <ilant@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>