[SCSI] iscsi: data digest calculation fix

From Wang Zhenyu:

data digest fix (the bug caused data corruption w/Wasabi StorageBuilder target)

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