ip6_tunnel: disable dst caching if tunnel is dual-stack

When an ip6_tunnel is in mode 'any', where the transport layer
protocol can be either 4 or 41, dst_cache must be disabled.

This is because xfrm policies might apply to only one of the two
protocols. Caching dst would cause xfrm policies for one protocol
incorrectly used for the other.

Signed-off-by: Eli Cooper <elicooper@gmx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>