mlxsw: spectrum_trap: Switch to use correct packet trap group

Some packet traps are currently exposed to user space as being member of
"l3_drops" trap group, but internally they are member of a different
group.

Switch these traps to use the correct group so that they are all subject
to the same policer, as exposed to user space.

Set the trap priority of packets trapped due to loopback error during
routing to the lowest priority. Such packets are not routed again by the
kernel and therefore should not mask other traps (e.g., host miss) that
should be routed.

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