tcp: inet6_csk_xmit() optimization

After prior patches, inet6_csk_xmit() can reuse inet->cork.fl.u.ip6
if __sk_dst_check() returns a valid dst.

Otherwise call inet6_csk_route_socket() to refresh inet->cork.fl.u.ip6
content and get a new dst.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260206173426.1638518-8-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>