NFS: Ensure O_DIRECT reports an error if the bytes read/written is 0

If the attempt to resend the I/O results in no bytes being read/written,
we must ensure that we report the error.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Fixes: 0a00b77b ("nfs: mirroring support for direct io")
Cc: stable@vger.kernel.org # v3.20+