tcp: add a drop_reason pointer to tcp_check_req()

We want to add new drop reasons for packets dropped in 3WHS in the
following patches.

tcp_rcv_state_process() has to set reason to TCP_FASTOPEN,
because tcp_check_req() will conditionally overwrite the drop_reason.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250301201424.2046477-2-edumazet@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>