NFS: nfs_readdir_search_for_cookie() don't mark as eof if cookie not found

If we're searching for a specific cookie, and it isn't found in the page
cache, we should try an uncached_readdir(). To do so, we return EBADCOOKIE,
but we don't set desc->eof.

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