Add comment about where we need to place this routine, and why.

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sys/cam/scsi/scsi_da.c

@@ -4200,6 +4200,12 @@ dadone(struct cam_periph *periph, union ccb *done_ccb)
 		if (LIST_EMPTY(&softc->pending_ccbs))
 			softc->flags |= DA_FLAG_WAS_OTAG;
 
+		/*
+		 * We need to call cam_iosched before we call biodone so that we
+		 * don't measure any activity that happens in the completion
+		 * routine, which in the case of sendfile can be quite
+		 * extensive.
+		 */
 		cam_iosched_bio_complete(softc->cam_iosched, bp, done_ccb);
 		xpt_release_ccb(done_ccb);
 		if (state == DA_CCB_DELETE) {