nfsd4: make sure set CB_PATH_DOWN sequence flag set

Make sure this is set whenever there is no callback channel.

If a client does not set up a callback channel at all, then it will get
this flag set from the very start.  That's OK, it can just ignore the
flag if it doesn't care.  If a client does care, I think it's better to
inform it of the problem as early as possible.
Reported-by: NRick Macklem <rmacklem@uoguelph.ca>
Signed-off-by: NJ. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4callback.c

@@ -986,7 +986,7 @@ static void nfsd4_process_cb_update(struct nfsd4_callback *cb)
 
 	err = setup_callback_client(clp, &conn, ses);
 	if (err) {
-		warn_no_callback_path(clp, err);
+		nfsd4_mark_cb_down(clp, err);
 		return;
 	}
 	/* Yay, the callback channel's back! Restart any callbacks: */