btrfs: add free space tree to lockdep classes

Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/disk-io.c

@@ -176,6 +176,7 @@ static struct btrfs_lockdep_keyset {
 	{ .id = BTRFS_TREE_RELOC_OBJECTID,	.name_stem = "treloc"	},
 	{ .id = BTRFS_DATA_RELOC_TREE_OBJECTID,	.name_stem = "dreloc"	},
 	{ .id = BTRFS_UUID_TREE_OBJECTID,	.name_stem = "uuid"	},
+	{ .id = BTRFS_FREE_SPACE_TREE_OBJECTID,	.name_stem = "free-space" },
 	{ .id = 0,				.name_stem = "tree"	},
 };