Increase cache locality by moving rx_dst_coookie next to sk->sk_rx_dst

This removes one or two cache line misses in IPv6 early demux (TCP/UDP)
Signed-off-by: NEric Dumazet <edumazet@google.com>
Acked-by: NSoheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: NJakub Kicinski <kuba@kernel.org>