In the long term, mars_aio will be replaced anyway because it
uses userspace concepts like ioctx.
Don't use the internal kthread_stop_nowait() anymore.
It is too cumbersome to catch up with upstream development.
The old code used mf->mf_max for correcting the file size, but that
was wrong for multiple writes in flight.
A really correct solution would have to remember all in-flight writes
and compute their minimum IO position. Since that would be too
costly, we just use the old size before any writes have started.
This might be too conservative for extremely high load patterns
(possible starvation problem). For now, take this and check whether
we really need higher effort.
