NFS: Ensure that all nfs lock contexts have a valid open context

Force the lock context to keep a reference to the parent open
context so that we can guarantee the validity of the latter.

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