btrfs: use common vfs LABEL ioctl definitions

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: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>