Improve kernel dumping reliability for busy ATA channels:
- Generate fake channel interrupts even if channel busy with previous
request to let it finish. Without this, dumping requests were just queued
and never processed.
- Drop pre-dump requests queue on dumping. ATA code, working in dumping
(interruptless) mode, unable to handle long request queue. Actually, to get
coherent dump we anyway should do as few unrelated actions as possible.