tcp: fix a signed-integer-overflow bug in tcp_add_backlog()

In the Linux kernel, the following vulnerability has been resolved: tcp: fix a signed-integer-overflow bug in tcp_add_backlog() The type of sk_rcvbuf and sk_sndbuf in struct sock is int, and in tcp_add_backlog(), the variable limit is caculated by adding sk_rcvbuf, sk_sndbuf and 64 * 1024, it may exceed the max value of int and overflow. This patch reduces the limit budget by halving the sndbuf to solve this issue since ACK packets are much smaller than the payload.