Exclude reserved iSCSI Target Transfer Tag.

RFC 7143 (11.7.4):
   The Target Transfer Tag values are not specified by this protocol,
   except that the value 0xffffffff is reserved and means that the
   Target Transfer Tag is not supplied.

MFC after:	1 month

sys/cam/ctl/ctl_frontend_iscsi.c

@@ -2693,6 +2693,10 @@ cfiscsi_datamove_out(union ctl_io *io)
 
 	target_transfer_tag =
 	    atomic_fetchadd_32(&cs->cs_target_transfer_tag, 1);
+	if (target_transfer_tag == 0xffffffff) {
+		target_transfer_tag =
+		    atomic_fetchadd_32(&cs->cs_target_transfer_tag, 1);
+	}
 	cdw = cfiscsi_data_wait_new(cs, io, bhssc->bhssc_initiator_task_tag,
 	    &target_transfer_tag);
 	if (cdw == NULL) {