selftests: fcnal: Test SO_DONTROUTE on TCP sockets.

Use nettest --{client,server}-dontroute to test the kernel behaviour
with TCP 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.

Client and server sockets are tested independently, so that we can
cover different TCP kernel paths.

SO_DONTROUTE also affects the syncookies path. So ipv4_tcp_dontroute()
is made to work with or without syncookies, to cover both paths.

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