ipv6: move tcp_ipv6_hash_secret and udp_ipv6_hash_secret to net_hotdata

Use a 32bit hole in "struct net_offload" to store
the remaining 32bit secrets used by TCPv6 and UDPv6.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20240306160031.874438-17-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>