NFS: O_DIRECT needs to use a completion

Now that we have aio writes, it is possible for dreq->outstanding to be
zero, but for the I/O not to have completed. Convert struct nfs_direct_req
to use a completion to signal when the I/O is done.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>