Fixing the DSACK code to properly clean up after sending a DSACK.

[rrs]

Summary

The DSACK RFC is quite clear that you only send a DSACK block
once for every received packet. The current implementation of DSACK
on FreeBSD has a flaw in that it only uses new data from a client to
drive it to clean up a DSACK block. This is incorrect, and instead
after a successful transmit any DSACK blocks should be purged.

wIthout this fix once you receive (as a server) a DSACK block
you may get it multiple times.

Test Plan

We have been now running with this at NF for at least a week and
validated that we no longer see the same DSACK sent out multiple
times without having received the same data multiple times.

[rscheff]

Sorry for the oversight; Indeed I tested this with continous data from the receiver only, not when the data direction changes :(