NFS: Fix atime revalidation in readdir()

NFSv3 will correctly update atime on a readdir call, so there is no need to
set the NFS_INO_INVALID_ATIME flag unless the call to nfs_refresh_inode()
fails.

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