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>