IPV6: dst_entry leak in ip4ip6_err.

Upstream commit: 9937ded8

The result of the ip_route_output is not assigned to skb. This means that
- it is leaked
- possible OOPS below dereferrencing skb->dst
- no ICMP message for this case

Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>