blktrace: Show requests without sector

Currently, blktrace will not show requests that don't have any data as
rq->__sector is initialized to -1 which is out of device range and thus
discarded by act_log_check(). This is most notably the case for cache
flush requests sent to the device. Fix the problem by making
blk_rq_trace_sector() return 0 for requests without initialized sector.
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/blktrace_api.h

@@ -116,7 +116,13 @@ extern void blk_fill_rwbs(char *rwbs, unsigned int op, int bytes);
 
 static inline sector_t blk_rq_trace_sector(struct request *rq)
 {
-	return blk_rq_is_passthrough(rq) ? 0 : blk_rq_pos(rq);
+	/*
+	 * Tracing should ignore starting sector for passthrough requests and
+	 * requests where starting sector didn't get set.
+	 */
+	if (blk_rq_is_passthrough(rq) || blk_rq_pos(rq) == (sector_t)-1)
+		return 0;
+	return blk_rq_pos(rq);
 }
 
 static inline unsigned int blk_rq_trace_nr_sectors(struct request *rq)