nfs: don't call nfs4_ff_layout_prepare_ds from ff_layout_get_ds_cred

All the callers already call that function before calling into here,
so it ends up being a no-op anyway.

Signed-off-by: Jeff Layton <jeff.layton@primarydata.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>