selftests: netdevsim: Add test for FIB offload API

Test various aspects of the FIB offload API on top of the netdevsim
implementation. Both good and bad flows are tested.

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