[SCSI] libsas: check dev->gone before submitting sata i/o

Head off doomed-to-fail i/o in sas_queuecommand before sending it down
the ata path.

Before:
sd 7:0:0:0: [sdd] Synchronizing SCSI cache
ata8: no sense translation for status: 0x00
ata8: translated ATA stat/err 0x00/00 to SCSI SK/ASC/ASCQ 0xb/00/00
ata8.00: device reported invalid CHS sector 0
ata8: status=0x00 { }
ata8: no sense translation for status: 0x00
ata8: translated ATA stat/err 0x00/00 to SCSI SK/ASC/ASCQ 0xb/00/00
ata8.00: device reported invalid CHS sector 0
ata8: status=0x00 { }
ata8: no sense translation for status: 0x00
ata8: translated ATA stat/err 0x00/00 to SCSI SK/ASC/ASCQ 0xb/00/00
ata8.00: device reported invalid CHS sector 0
ata8: status=0x00 { }
sd 7:0:0:0: [sdd]  Result: hostbyte=DID_OK driverbyte=DRIVER_SENSE
sd 7:0:0:0: [sdd]  Sense Key : Aborted Command [current] [descriptor]
sd 7:0:0:0: [sdd]  Add. Sense: No additional sense information
sd 7:0:0:0: [sdd] Stopping disk

After:
sd 9:0:0:0: [sdd] Synchronizing SCSI cache
sd 9:0:0:0: [sdd]  Result: hostbyte=DID_BAD_TARGET driverbyte=DRIVER_OK
sd 9:0:0:0: [sdd] Stopping disk
sd 9:0:0:0: [sdd] START_STOP FAILED
sd 9:0:0:0: [sdd]  Result: hostbyte=DID_BAD_TARGET driverbyte=DRIVER_OK

This is a cosmetic change as sata i/o can still leak to a gone device,
but this addresses the nominal hotplug case when releasing the target.

Acked-by: Jack Wang <jack_wang@usish.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: James Bottomley <jbottomley@parallels.com>