Extend q->prep_rq_fn() to return one of three values:

 o BLKPREP_OK: request is good, return it
 o BLKPREP_KILL: request is bad, end it completely
 o BLKPREP_DEFER: request is good, but we can't take it now

We maintain compatability with old prep functions (if any, outside of
ide-cd). This change is needed or SCSI to use prep function for command
init, if sg table allocation fails we can just defer the request.