ipv6: add daddr/final storage in struct ipv6_pinfo

ipv6: add daddr/final storage in struct ipv6_pinfo

After commit b409a7f7176b ("ipv6: colocate inet6_cork in
inet_cork_full") we have room in ipv6_pinfo to hold daddr/final
in case they need to be populated in fl6_update_dst() calls.

This will allow stack canary removal in IPv6 tx fast paths.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260206173426.1638518-2-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>