ipv6: move inet6_ehash_secret and udp6_ehash_secret into net_hotdata

"struct inet6_protocol" has a 32bit hole in 32bit arches.

Use it to store the 32bit secret used by UDP and TCP,
to increase cache locality in rx path.

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/20240306160031.874438-16-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>