dio: Convert direct_IO to use iov_iter

dio: Convert direct_IO to use iov_iter

Change the direct_IO aop to take an iov_iter argument rather than an iovec.
This will get passed down through most filesystems so that only the
__blockdev_direct_IO helper need be aware of whether user or kernel memory
is being passed to the function.

Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com>