This fixes a bug that was introduced in #11258 where the handling of the
per-client limit didn't properly account for the fact that the gVisor
TCP forwarder will return 'true' to indicate that it's handled a
duplicate SYN packet, but not launch the handler goroutine.
In such a case, we neither decremented our per-client limit in the
wrapper function, nor did we do so in the handler function, leading to
our per-client limit table slowly filling up without bound.
Fix this by doing the same duplicate-tracking logic that the TCP
forwarder does so we can detect such cases and appropriately decrement
our in-flight counter.
Updates tailscale/corp#12184
Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: Ib6011a71d382a10d68c0802593f34b8153d06892
7429e8912a