DTrace probe gw__write__done is never called.

[leftwo]

Now that we fast-ack writes (and write_unwritten), we no longer hit the gw__write__done DTrace probe in it's current location.

Figure out if we want something better, or maybe just delete the probe and re-name the current one.

There are a number of dtrace scripts that try to measure things based on the gw__write__done and we should update those to.. do something different.

[leftwo]

Fixed in #1617

the gw__write__done probe is now hit.