PDU use serial32 arithmetic for cmdsn, maxcmdsn and expcmdsn.

RFC3720 says that cmdsn comparison must be done using
serial32 arithmetic. This will definetly avoid a deadlock
if cmdsn wraps from 2^32-1 to 0.

Signed-off-by: Peter Lieven <pl@kamp.de>

lib/task_mgmt.c

@@ -88,7 +88,7 @@ iscsi_process_task_mgmt_reply(struct iscsi_context *iscsi, struct iscsi_pdu *pdu
 	response = in->hdr[2];
 
 	maxcmdsn = scsi_get_uint32(&in->hdr[32]);
-	if (maxcmdsn > iscsi->maxcmdsn) {
+	if (iscsi_serial32_compare(maxcmdsn,iscsi->maxcmdsn) > 0) {
 		iscsi->maxcmdsn = maxcmdsn;
 	}