mlxsw: Add specific trap for packets routed via invalid nexthops

Currently, mlxsw does not differentiate between these two cases of
routes with invalid nexthops:

1. Nexthops whose nexthop device is a mlxsw upper (has a RIF), but whose
neighbour could not be resolved

2. Nexthops whose nexthop device is not a mlxsw upper (e.g., management
interface)

Up until now this did not matter and mlxsw trapped packets for both
cases using the same trap ID. However, packets that should have been
routed in hardware (case 1), but incurred a problem are considered
exceptions and should be reported to the user. The two cases should
therefore be split between two different trap IDs.

Allocate a new adjacency entry during initialization and upon the
insertion of the first route with an invalid mlxsw nexthop, program this
entry to discard packets. Packets hitting this entry will be reported
using new trap ID - "DISCARD_ROUTER3".

In the future, the entry could be written during initialization, but
currently firmware requires a valid RIF, which is not available at this
stage.

Signed-off-by: Amit Cohen <amitc@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>