ipv6: inet6_csk_xmit() and inet6_csk_update_pmtu() use inet->cork.fl.u.ip6

ipv6: inet6_csk_xmit() and inet6_csk_update_pmtu() use inet->cork.fl.u.ip6

Convert inet6_csk_route_socket() to use np->final instead of an
automatic variable to get rid of a stack canary.

Convert inet6_csk_xmit() and inet6_csk_update_pmtu() to use
inet->cork.fl.u.ip6 instead of @fl6 automatic variable.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260206173426.1638518-5-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>