I lifted the btrfs label get/set ioctls to the vfs some time ago, but
never followed up to use those common definitions directly in btrfs.

This patch does that.
Signed-off-by: NEric Sandeen <sandeen@redhat.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>