Revert "dio: Convert direct_IO to use iov_iter"

This reverts commit b396e316d65d07f3f3c4d6cc9cc2cef19bfb1020.
Revert "iov_iter: let callers extract iovecs and bio_vecs"

This reverts commit cba4e486a69e6417598f1f0a729d7f6146394d35.