netdevsim: Add dummy implementation for nexthop offload

Implement dummy nexthop "offload" in the driver by storing currently
"programmed" nexthops in a hash table. Each nexthop in the hash table is
marked with "trap" indication and increments the nexthops resource
occupancy.

This will later allow us to test the nexthop offload API on top of
netdevsim.

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