md/raid10: fix FailFast test for wrong device

We need to test FailFast flag for replacement device here
since the set up for writing is for the replacement, so we
need fix it like:

- if (test_bit(FailFast, &conf->mirrors[d].rdev->flags))
+ if (test_bit(FailFast, &conf->mirrors[d].replacement->flags))

Since commit f90145f317ef ("md/raid10: add rcu protection
to rdev access in raid10_sync_request.") had added the rcu
protection for the part, so let's extend the range protected
by rcu and use rdev directly.

Fixes: 1919cbb ("md/raid10: add failfast handling for writes.")
Reviewed-by: NeilBrown <neilb@suse.com>
Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>

drivers/md/raid10.c

@@ -3295,7 +3295,7 @@ static sector_t raid10_sync_request(struct mddev *mddev, sector_t sector_nr,
 			biolist = bio;
 			bio->bi_end_io = end_sync_read;
 			bio_set_op_attrs(bio, REQ_OP_READ, 0);
-			if (test_bit(FailFast, &conf->mirrors[d].rdev->flags))
+			if (test_bit(FailFast, &rdev->flags))
 				bio->bi_opf |= MD_FAILFAST;
 			bio->bi_iter.bi_sector = sector + rdev->data_offset;
 			bio->bi_bdev = rdev->bdev;
@@ -3307,7 +3307,6 @@ static sector_t raid10_sync_request(struct mddev *mddev, sector_t sector_nr,
 				continue;
 			}
 			atomic_inc(&rdev->nr_pending);
-			rcu_read_unlock();
 
 			/* Need to set up for writing to the replacement */
 			bio = r10_bio->devs[i].repl_bio;
@@ -3318,11 +3317,12 @@ static sector_t raid10_sync_request(struct mddev *mddev, sector_t sector_nr,
 			biolist = bio;
 			bio->bi_end_io = end_sync_write;
 			bio_set_op_attrs(bio, REQ_OP_WRITE, 0);
-			if (test_bit(FailFast, &conf->mirrors[d].rdev->flags))
+			if (test_bit(FailFast, &rdev->flags))
 				bio->bi_opf |= MD_FAILFAST;
 			bio->bi_iter.bi_sector = sector + rdev->data_offset;
 			bio->bi_bdev = rdev->bdev;
 			count++;
+			rcu_read_unlock();
 		}
 
 		if (count < 2) {