tcp/dccp: drop SYN packets if accept queue is full

[ Upstream commit: 5ea8ea2cb7f1d0db15762c9b0bb9e7330425a071 ]

Per listen(fd, backlog) rules, there is really no point accepting a SYN,
sending a SYNACK, and dropping the following ACK packet if accept queue
is full, because application is not draining accept queue fast enough.

This behavior is fooling TCP clients that believe they established a
flow, while there is nothing at server side. They might then send about
10 MSS (if using IW10) that will be dropped anyway while server is under
stress.

Change-Id: I6e7b8801513265523e7092e0e5788cf45f06d943
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
Git-commit: 5ea8ea2cb7f1d0db15762c9b0bb9e7330425a071
Git-repo: https://android.googlesource.com/kernel/common/


Signed-off-by: Chinmay Agarwal <chinagar@codeaurora.org>