Subvolume directory inodes can't have ACLs.

Cc: <stable@vger.kernel.org> # 4.9.x
Signed-off-by: NOmar Sandoval <osandov@fb.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NChris Mason <clm@fb.com>