nfs/localio: remove unecessary ENOTBLK handling in DIO WRITE support

nfs/localio: remove unecessary ENOTBLK handling in DIO WRITE support

Each filesystem is meant to fallback to retrying DIO in terms buffered
IO when it might encounter -ENOTBLK when issuing DIO (which can happen
if the VFS cannot invalidate the page cache).

So NFS doesn't need special handling for -ENOTBLK.

Also, explicitly initialize a couple DIO related iocb members rather
than simply rely on data structure zeroing.

Fixes: c817248fc831 ("nfs/localio: add proper O_DIRECT support for READ and WRITE")
Reported-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>