[SCSI] sas_ata: Assign sas_task to scsi_cmnd to enable EH for ATA devices

The SATL should connect the scsi_cmnd to the sas_task (despite the presence
of libata) so that requests to abort scsi_cmnds headed to the ATA device
can be processed by the EH and aborted correctly.  The abort status should
still be propagated from sas -> ata -> scsi.

Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>