Btrfs, replace: write raid56 parity into the replace target device

This function reused the code of parity scrub, and we just write
the right parity or corrected parity into the target device before
the parity scrub end.

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
index 7f95afc..0ae837f 100644
--- a/fs/btrfs/scrub.c
+++ b/fs/btrfs/scrub.c
@@ -2714,7 +2714,7 @@
 		goto out;
 
 	length = sparity->logic_end - sparity->logic_start + 1;
-	ret = btrfs_map_sblock(sctx->dev_root->fs_info, REQ_GET_READ_MIRRORS,
+	ret = btrfs_map_sblock(sctx->dev_root->fs_info, WRITE,
 			       sparity->logic_start,
 			       &length, &bbio, 0, &raid_map);
 	if (ret || !bbio || !raid_map)