Merge branch 'tcp-clear-tcp_sk-sk-fastopen_rsk-in-tcp_disconnect'

Kuniyuki Iwashima says:

====================
tcp: Clear tcp_sk(sk)->fastopen_rsk in tcp_disconnect().

syzbot reported a warning in tcp_retransmit_timer() for TCP Fast
Open socket.

Patch 1 fixes the issue and Patch 2 adds a test for the scenario.
====================

Link: https://patch.msgid.link/20250915175800.118793-1-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>