Ok here's an updated patch, it changes the barriers around, updates
comments, and gets rid of the waited check in __get_request_wait.  It is
still a combined patch with fix_pausing, queue_full and latency stats,
mostly because I want to make really sure any testers are using all
three.

So, if someone who saw io stalls in 2.4.21-rc could give this a try, I'd
be grateful.  If you still see stalls with this applied, run elvtune
/dev/xxx and send along the resulting console output.

-chris