NFSv4.1/pnfs: Mark the layout stateid invalid when all segments are removed

According to RFC5661, section 12.5.3. the layout stateid is no longer
valid once the client no longer holds any layout segments. Ensure that
we mark it invalid.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>