We cannot sensibly support O_DIRECT reads or writes when all writes are
journalled.

This is because the VFS explicitly avoids syncing the file metadata during
O_DIRECT reads and writes.  ext3 with journalled data will leave pending
changes in memory and they will overwrite the results of O_DIRECT writes, and
O_DIRECT reads will not return the latest data.

Setting the a_op to null will cause opens and fcntl(F_SETFL) to return
-EINVAL if O_DIRECT is requested.