tcp: rstreason: fully support in tcp_check_req()

tcp: rstreason: fully support in tcp_check_req()

We're going to send an RST due to invalid syn packet which is already
checked whether 1) it is in sequence, 2) it is a retransmitted skb.

As RFC 793 says, if the state of socket is not CLOSED/LISTEN/SYN-SENT,
then we should send an RST when receiving bad syn packet:
"fourth, check the SYN bit,...If the SYN is in the window it is an
error, send a reset"

Signed-off-by: Jason Xing <kernelxing@tencent.com>
Link: https://lore.kernel.org/r/20240510122502.27850-6-kerneljasonxing@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>