tcp: populate inet->cork.fl.u.ip6 in tcp_v6_connect()

tcp: populate inet->cork.fl.u.ip6 in tcp_v6_connect()

Instead of using private @fl6 and @final variables
use respectively inet->cork.fl.u.ip6 and np->final.

As explained in commit 85d05e281712 ("ipv6: change inet6_sk_rebuild_header()
to use inet->cork.fl.u.ip6"):

TCP v6 spends a good amount of time rebuilding a fresh fl6 at each
transmit in inet6_csk_xmit()/inet6_csk_route_socket().

TCP v4 caches the information in inet->cork.fl.u.ip4 instead.

After this patch, active TCP ipv6 flows have correctly initialized
inet->cork.fl.u.ip6 structure.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260206173426.1638518-6-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>