[SCSI] libiscsi: regression: fix header digest errors

This fixes a regression introduced with this commit:

commit d3305f34
Author: Mike Christie <michaelc@cs.wisc.edu>
Date:   Thu Aug 20 15:10:58 2009 -0500

    [SCSI] libiscsi: don't increment cmdsn if cmd is not sent

in 2.6.32.

When I moved the hdr->cmdsn after init_task, I added
a bug when header digests are used. The problem is
that the LLD may calculate the header digest in init_task,
so if we then set the cmdsn after the init_task call we
change what the digest will be calculated by the target.
Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Cc: Stable Tree <stable@kernel.org>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>

drivers/scsi/libiscsi.c

@@ -471,12 +471,12 @@ static int iscsi_prep_scsi_cmd_pdu(struct iscsi_task *task)
 
 	WARN_ON(hdrlength >= 256);
 	hdr->hlength = hdrlength & 0xFF;
+	hdr->cmdsn = task->cmdsn = cpu_to_be32(session->cmdsn);
 
 	if (session->tt->init_task && session->tt->init_task(task))
 		return -EIO;
 
 	task->state = ISCSI_TASK_RUNNING;
-	hdr->cmdsn = task->cmdsn = cpu_to_be32(session->cmdsn);
 	session->cmdsn++;
 
 	conn->scsicmd_pdus_cnt++;