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>

Documentation/networking/devlink/devlink-trap.rst

@@ -476,6 +476,10 @@ be added to the following table:
    * - ``esp_parsing``
      - ``drop``
      - Traps packets dropped due to an error in the ESP header parsing
+   * - ``blackhole_nexthop``
+     - ``drop``
+     - Traps packets that the device decided to drop in case they hit a
+       blackhole nexthop
 
 Driver-specific Packet Traps
 ============================