NFS: Don't open code clearing of delegation state

Add a helper for the case when the nfs4 open state has been set to use
a delegation stateid, and we want to revert to using the open stateid.

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