mlxsw: spectrum_trap: Add blackhole_nexthop trap

Register with devlink the blackhole_nexthop trap so that mlxsw will be
able to report packets dropped due to a blackhole nexthop.

The internal trap identifier is "DISCARD_ROUTER3", which traps packets
dropped in the adjacency table.
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/mellanox/mlxsw/spectrum_trap.c

@@ -1007,6 +1007,12 @@ static const struct mlxsw_sp_trap_item mlxsw_sp_trap_items_arr[] = {
 					     false),
 		},
 	},
+	{
+		.trap = MLXSW_SP_TRAP_DROP(BLACKHOLE_NEXTHOP, L3_DROPS),
+		.listeners_arr = {
+			MLXSW_SP_RXL_DISCARD(ROUTER3, L3_DISCARDS),
+		},
+	},
 };
 
 static struct mlxsw_sp_trap_policer_item *