vti: Use the tunnel mark for lookup in the error handlers.

We need to use the mark we get from the tunnels o_key to
lookup the right vti state in the error handlers. This patch
ensures that.

Fixes: df3893c1 ("vti: Update the ipv4 side to use it's own receive hook.")
Fixes: fa9ad96d ("vti6: Update the ipv6 side to use its own receive hook.")
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>