nfsd: release the legacy reclaimable clients list in grace_done

The current code holds on to this list until nfsd is shut down, but it's
never touched once the grace period ends. Release that memory back into
the wild when the grace period ends.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>