selftests: forwarding: Add helpers and tests for FIB offload

Implement a set of common helpers and tests for FIB offload that can be
used by multiple drivers to check their FIB offload implementations.

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