pNFS/flexfiles: Handle expired layout segments in ff_layout_initiate_commit()

If the layout has expired due to a fencing event, then we should not
attempt to commit to the DS.

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