NFS: Switch readdir to using iterate_shared()

Now that the page cache locking is repaired, we should be able to
switch to using iterate_shared() for improved concurrency when
doing readdir().

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>