mlxsw: spectrum: Use different trap group for externally routed packets

Cited commit mistakenly removed the trap group for externally routed
packets (e.g., via the management interface) and grouped locally routed
and externally routed packet traps under the same group, thereby
subjecting them to the same policer.

This can result in problems, for example, when FRR is restarted and
suddenly all transient traffic is trapped to the CPU because of a
default route through the management interface. Locally routed packets
required to re-establish a BGP connection will never reach the CPU and
the routing tables will not be re-populated.

Fix this by using a different trap group for externally routed packets.

Fixes: 8110668e ("mlxsw: spectrum_trap: Register layer 3 control traps")
Reported-by: Alex Veber <alexve@mellanox.com>
Tested-by: Alex Veber <alexve@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>