btrfs: show correct warning if can't read data reloc tree

If a filesystem is missing its data reloc tree, we get something like
this in dmesg:

  BTRFS warning (device loop11): failed to read root (objectid=4): -2
  BTRFS error (device loop11): open_ctree failed: -2

objectid is BTRFS_DEV_TREE_OBJECTID, but this should actually be the
value of BTRFS_DATA_RELOC_TREE_OBJECTID.

btrfs_read_roots() prints location.objectid on failure, but this isn't
set when reading the data reloc tree. Set location.objectid to the
correct value on failure, so that the error message makes sense.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Mark Harmstone <mark@harmstone.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: maharmstone

fs/btrfs/disk-io.c

@@ -2255,6 +2255,7 @@ static int btrfs_read_roots(struct btrfs_fs_info *fs_info)
 				 BTRFS_DATA_RELOC_TREE_OBJECTID, true);
 	if (IS_ERR(root)) {
 		if (!btrfs_test_opt(fs_info, IGNOREBADROOTS)) {
+			location.objectid = BTRFS_DATA_RELOC_TREE_OBJECTID;
 			ret = PTR_ERR(root);
 			goto out;
 		}