ip_tunnel: fix possible rtable leak

ip_rt_put(rt) is always called in "error" branches above, but was missed in
skb_cow_head branch. As rt is not yet bound to skb here we have to release it by
hand.

Signed-off-by: Dmitry Popov <ixaphire@qrator.net>
Signed-off-by: David S. Miller <davem@davemloft.net>