vxlan: release rt when found circular route

Otherwise causing dst memory leakage.
Have Checked all other type tunnel device transmit implementation,
no such things happens anymore.

Signed-off-by: Fan Du <fan.du@windriver.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>