btrfs: scrub: Don't append on-disk pages for raid56 scrub

In the following situation, scrub will calculate wrong parity to
overwrite the correct one:

RAID5 full stripe:

Before
|     Dev 1      |     Dev  2     |     Dev 3     |
| Data stripe 1  | Data stripe 2  | Parity Stripe |
--------------------------------------------------- 0
| 0x0000 (Bad)   |     0xcdcd     |     0x0000    |
--------------------------------------------------- 4K
|     0xcdcd     |     0xcdcd     |     0x0000    |
...
|     0xcdcd     |     0xcdcd     |     0x0000    |
--------------------------------------------------- 64K

After scrubbing dev3 only:

|     Dev 1      |     Dev  2     |     Dev 3     |
| Data stripe 1  | Data stripe 2  | Parity Stripe |
--------------------------------------------------- 0
| 0xcdcd (Good)  |     0xcdcd     | 0xcdcd (Bad)  |
--------------------------------------------------- 4K
|     0xcdcd     |     0xcdcd     |     0x0000    |
...
|     0xcdcd     |     0xcdcd     |     0x0000    |
--------------------------------------------------- 64K

The reason is that after raid56 read rebuild rbio->stripe_pages are all
correctly recovered (0xcd for data stripes).

However when we check and repair parity in
scrub_parity_check_and_repair(), we will append pages in sparity->spages
list to rbio->bio_pages[], which contains old on-disk data.

And when we submit parity data to disk, we calculate parity using
rbio->bio_pages[] first, if rbio->bio_pages[] not found, then fallback
to rbio->stripe_pages[].

The patch fix it by not appending pages from sparity->spages.
So finish_parity_scrub() will use rbio->stripe_pages[] which is correct.
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Reviewed-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/scrub.c

@@ -2769,7 +2769,6 @@ static void scrub_parity_check_and_repair(struct scrub_parity *sparity)
 	struct btrfs_fs_info *fs_info = sctx->fs_info;
 	struct bio *bio;
 	struct btrfs_raid_bio *rbio;
-	struct scrub_page *spage;
 	struct btrfs_bio *bbio = NULL;
 	u64 length;
 	int ret;
@@ -2799,9 +2798,6 @@ static void scrub_parity_check_and_repair(struct scrub_parity *sparity)
 	if (!rbio)
 		goto rbio_out;
 
-	list_for_each_entry(spage, &sparity->spages, list)
-		raid56_add_scrub_pages(rbio, spage->page, spage->logical);
-
 	scrub_pending_bio_inc(sctx);
 	raid56_parity_submit_scrub_rbio(rbio);
 	return;