Btrfs-progs: add type root to label property

Btrfs-progs: add type root to label property

So that we can get the label of a mounted filesystem.

Before this change:

        $ btrfs prop get /mnt/btrfs label
        ERROR: object is not compatible with property

        $ btrfs prop get /dev/sdb3 label
        ERROR: dev /dev/sdb3 is mounted, use mount point
        ERROR: failed to set/get property for object.

After this change:

        $ btrfs prop get /mnt/btrfs label
        label=foobar

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>