NFS: Ensure that we update the readdir filp->f_pos correctly

If we're doing a search by readdir cookie, we need to ensure that the
resulting f_pos is updated. To do so, we need to update the
desc->current_index, in the same way that we do in the search by
file offset case.

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