NFS: Clear NFS_DELEGATION_RETURN_IF_CLOSED when the delegation is returned

If a delegation is marked as needing to be returned when the file is
closed, then don't clear that marking until we're ready to return
it.

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