selftests: fcnal: Test SO_DONTROUTE on UDP sockets.

Use nettest --client-dontroute to test the kernel behaviour with UDP
sockets having the SO_DONTROUTE option. Sending packets to a neighbour
(on link) host, should work. When the host is behind a router, sending
should fail.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>