stat: avoid hang with race in stat thread exit and rusage update

Signed-off-by: Jens Axboe <axboe@fb.com>
Consolidate disk util, eta, and status check thread

We don't need two, we can just have one do everything.

Signed-off-by: Jens Axboe <axboe@fb.com>
diskutil: get rid of disk_util_start_exit()

No need to wrap this in a function, just set the exit variable.

Signed-off-by: Jens Axboe <axboe@fb.com>