block: Fix fsync always failed if once failed

We do test with inject error fault base on v4.19, after test some time we found
sync /dev/sda always failed.
[root@localhost] sync /dev/sda
sync: error syncing '/dev/sda': Input/output error

scsi log as follows:
[19069.812296] sd 0:0:0:0: [sda] tag#64 Send: scmd 0x00000000d03a0b6b
[19069.812302] sd 0:0:0:0: [sda] tag#64 CDB: Synchronize Cache(10) 35 00 00 00 00 00 00 00 00 00
[19069.812533] sd 0:0:0:0: [sda] tag#64 Done: SUCCESS Result: hostbyte=DID_OK driverbyte=DRIVER_OK
[19069.812536] sd 0:0:0:0: [sda] tag#64 CDB: Synchronize Cache(10) 35 00 00 00 00 00 00 00 00 00
[19069.812539] sd 0:0:0:0: [sda] tag#64 scsi host busy 1 failed 0
[19069.812542] sd 0:0:0:0: Notifying upper driver of completion (result 0)
[19069.812546] sd 0:0:0:0: [sda] tag#64 sd_done: completed 0 of 0 bytes
[19069.812549] sd 0:0:0:0: [sda] tag#64 0 sectors total, 0 bytes done.
[19069.812564] print_req_error: I/O error, dev sda, sector 0

ftrace log as follows:
 rep-306069 [007] .... 19654.923315: block_bio_queue: 8,0 FWS 0 + 0 [rep]
 rep-306069 [007] .... 19654.923333: block_getrq: 8,0 FWS 0 + 0 [rep]
 kworker/7:1H-250   [007] .... 19654.923352: block_rq_issue: 8,0 FF 0 () 0 + 0 [kworker/7:1H]
 <idle>-0     [007] ..s. 19654.923562: block_rq_complete: 8,0 FF () 18446744073709551615 + 0 [0]
 <idle>-0     [007] d.s. 19654.923576: block_rq_complete: 8,0 WS () 0 + 0 [-5]

As 8d699663 introduce 'fq->rq_status', this data only update when 'flush_rq'
reference count isn't zero. If flush request once failed and record error code
in 'fq->rq_status'. If there is no chance to update 'fq->rq_status',then do fsync
will always failed.
To address this issue reset 'fq->rq_status' after return error code to upper layer.

Fixes: 8d699663("block: fix null pointer dereference in blk_mq_rq_timed_out()")
Signed-off-by: Ye Bin <yebin10@huawei.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20211129012659.1553733-1-yebin10@huawei.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-flush.c

@@ -242,8 +242,10 @@ static void flush_end_io(struct request *flush_rq, blk_status_t error)
 	 * avoiding use-after-free.
 	 */
 	WRITE_ONCE(flush_rq->state, MQ_RQ_IDLE);
-	if (fq->rq_status != BLK_STS_OK)
+	if (fq->rq_status != BLK_STS_OK) {
 		error = fq->rq_status;
+		fq->rq_status = BLK_STS_OK;
+	}
 
 	if (!q->elevator) {
 		flush_rq->tag = BLK_MQ_NO_TAG;