[SCSI] iscsi: preemt fix and cleanup

From: zhenyu.z.wang@intel.com

Delay the head digest update until xmit time, like data digest update.
[To make things cleaner and avoid prempt bug]

Signed-off-by: Alex Aizman <itn780@yahoo.com>
Signed-off-by: Dmitry Yusupov <dmitry_yus@yahoo.com>
Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>