[IPV6] ip6_tunnel: release cached dst on change of tunnel params

The included patch fixes ip6_tunnel to release the cached dst entry
when the tunnel parameters (such as tunnel endpoints) are changed so
they are used immediatly for the next encapsulated packets.

Signed-off-by: Hugo Santos <hsantos@av.it.pt>
Acked-by: Ville Nuorvala <vnuorval@tcs.hut.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>