selftests: Add SO_DONTROUTE option to nettest.

Add --client-dontroute and --server-dontroute options to nettest. They
allow to set the SO_DONTROUTE option to the client and server sockets
respectively. This will be used by the following patches to test
the SO_DONTROUTE kernel behaviour with TCP and UDP.

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