devlink: Add blackhole_nexthop trap

Add a packet trap to report packets that were dropped due to a
blackhole nexthop.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>