tcp: improve consistency of syncache_respond() failure handling

[tuexen]

Summary

When the initial sending of the SYN ACK segment using syncache_respond() fails, it is handled as a permanent error. To improve consistency, apply this policy in all cases, where syncache_respond() is called. These include

• timer based retransmissions of the SYN ACK
• retransmitting a SYN ACK in response to a SYN retransmission
• sending of challenge ACKs in response to received RST segments

In these cases, fall back to SYN cookies, if enabled. While there, also improve consistency of the TCP stats counters.
This was discussed at the last FreeBSD transport VC.

[cc]

Besides, I am wondering if TCPSTAT_INC(tcps_sndacks) and TCPSTAT_INC(tcps_sndtotal) consistency can also be improved after successful syncache_respond().

[tuexen]

Improve consistency of the TCP stats counters as suggested by cc@.

[tuexen]

In D46428#1060788, @cc wrote:

Besides, I am wondering if TCPSTAT_INC(tcps_sndacks) and TCPSTAT_INC(tcps_sndtotal) consistency can also be improved after successful syncache_respond().

Thanks for the suggestion. Done in the latest version of the patch.