kernel - Do not invalidate the vnode+children in the NFS core on ESTALE

* When NFS encounters ESTALE it would try to invalidate the related
  namecache topology in the core socket code, requiring an exclusive
  lock on related namecache entries.  This is not compatible with
  shared namecache locking because it can occur as part of any operation,
  even simple GETATTRs.

* Remove the invalidation for now.  The kernel already has a high-level
  solution in place for terminal nodes but we will also need one for
  traversals.