ipv6/ip6_gre: fix error case handling in ip6gre_tunnel_xmit()

ip6gre_tunnel_xmit() is leaking the skb when we hit this error branch,
and the -1 return value from this function is bogus. Use the error
handling we already have in place in ip6gre_tunnel_xmit() for this error
case to fix this.

Signed-off-by: Tommi Rantala <tt.rantala@gmail.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>