grace: replace BUG_ON by WARN_ONCE in exit_net hook

BugLink: http://bugs.launchpad.net/bugs/1755509

[ Upstream commit b8722857 ]
Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

fs/nfs_common/grace.c

@@ -104,7 +104,9 @@ grace_exit_net(struct net *net)
 {
 	struct list_head *grace_list = net_generic(net, grace_net_id);
 
-	BUG_ON(!list_empty(grace_list));
+	WARN_ONCE(!list_empty(grace_list),
+		  "net %x %s: grace_list is not empty\n",
+		  net->ns.inum, __func__);
 }
 
 static struct pernet_operations grace_net_ops = {