nfsd: don't use locks_in_grace to determine whether to call nfs4_grace_end

nfsd: don't use locks_in_grace to determine whether to call nfs4_grace_end

It's possible that lockd or another lock manager might still be on the
list after we call nfsd4_end_grace. If the laundromat thread runs
again at that point, then we could end up calling nfsd4_end_grace more
than once.

That's not only inefficient, but calling nfsd4_recdir_purge_old more
than once could be problematic. Fix this by adding a new global
"grace_ended" flag and use that to determine whether we've already
called nfsd4_grace_end.

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