scsi: target: tcmu: Use priv pointer in se_cmd

We initialize and clean up the se_cmd's priv pointer under cmd_ring_lock to
point to the corresponding tcmu_cmd.

In the patch that implements tmr_notify callback in tcmu we will use the
priv pointer.

Link: https://lore.kernel.org/r/20200726153510.13077-4-bstroesser@ts.fujitsu.com
Reviewed-by: Mike Christie <michael.christie@oracle.com>
Signed-off-by: Bodo Stroesser <bstroesser@ts.fujitsu.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>