vti6: Fix memory leak of skb if input policy check fails

The vti6_rcv function performs some tests on the retrieved tunnel
including checking the IP protocol, the XFRM input policy, the
source and destination address.

In all but one places the skb is released in the error case. When
the input policy check fails the network packet is leaked.

Using the same goto-label discard in this case to fix this problem.

Fixes: ed1efb2a ("ipv6: Add support for IPsec virtual tunnel interfaces")
Signed-off-by: Torsten Hilbrich <torsten.hilbrich@secunet.com>
Reviewed-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>