Problem:
   During the libata CD-ROM stress test, sometimes the "BUG: timeout
without command" error is seen.

Root cause:
  Unexpected interrupt occurs after the ata_qc_complete() is called,
but before the SCSI error handler.  The interrupt handler is invoked
before the SCSI error handler, and it clears the command by calling
ata_qc_complete() again.  Later when the SCSI error handler is run,
the ata_queued_cmd is already gone, causing the "BUG: timeout without
command" error.

Changes:
  - Use the ATA_QCFLAG_ACTIVE flag to prevent the interrupt handler
from completing the command twice, before the scsi_error_handler.
Signed-off-by: Albert Lee <albertcc@tw.ibm.com>