Btrfs: Set real mirror number for read operation on RAID0/5/6

We need real mirror number for RAID0/5/6 when reading data, or if read error
happens, we would pass 0 as the number of the mirror on which the io error
happens. It is wrong and would cause the filesystem read the data from the
corrupted mirror again.
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/volumes.c

@@ -5073,6 +5073,8 @@ static int __btrfs_map_block(struct btrfs_fs_info *fs_info, int rw,
 			num_stripes = min_t(u64, map->num_stripes,
 					    stripe_nr_end - stripe_nr_orig);
 		stripe_index = do_div(stripe_nr, map->num_stripes);
+		if (!(rw & (REQ_WRITE | REQ_DISCARD | REQ_GET_READ_MIRRORS)))
+			mirror_num = 1;
 	} else if (map->type & BTRFS_BLOCK_GROUP_RAID1) {
 		if (rw & (REQ_WRITE | REQ_DISCARD | REQ_GET_READ_MIRRORS))
 			num_stripes = map->num_stripes;
@@ -5176,6 +5178,9 @@ static int __btrfs_map_block(struct btrfs_fs_info *fs_info, int rw,
 			/* We distribute the parity blocks across stripes */
 			tmp = stripe_nr + stripe_index;
 			stripe_index = do_div(tmp, map->num_stripes);
+			if (!(rw & (REQ_WRITE | REQ_DISCARD |
+				    REQ_GET_READ_MIRRORS)) && mirror_num <= 1)
+				mirror_num = 1;
 		}
 	} else {
 		/*