udp6: set rx_dst_cookie on rx_dst updates

Currently, in the udp6 code, the dst cookie is not initialized/updated
concurrently with the RX dst used by early demux.

As a result, the dst_check() in the early_demux path always fails,
the rx dst cache is always invalidated, and we can't really
leverage significant gain from the demux lookup.

Fix it adding udp6 specific variant of sk_rx_dst_set() and use it
to set the dst cookie when the dst entry is really changed.

The issue is there since the introduction of early demux for ipv6.

Fixes: 5425077d ("net: ipv6: Add early demux handler for UDP unicast")
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>