latest changes to ata driver increases stability

[Sean McNeil]

Just wanted to show my appreciation for the latest work checked in by
Soren.  I've been plagued with system hangs on my amd64 system for quite
a while and the latest changes have (so far) eliminated them. The
PREEMPTION caused many failures, but the last myserious cause would seem
to have been ATA.  As an added bonus, by DVD writer is working again.

Many thanks,
Sean