nexthop: Emit a notification when a nexthop group is reduced

When a single nexthop is deleted, the configuration of all the groups
using the nexthop is effectively modified. In this case, emit a
notification in the nexthop notification chain for each modified group
so that listeners would not need to keep track of which nexthops are
member in which groups.

In the rare cases where the notification fails, emit an error to the
kernel log. This is done by allocating extack on the stack and printing
the error logged by the listener that rejected the notification.

Changes since RFC:
* Allocate extack on the stack
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/nexthop.c

@@ -892,9 +892,10 @@ static void remove_nh_grp_entry(struct net *net, struct nh_grp_entry *nhge,
 {
 	struct nh_grp_entry *nhges, *new_nhges;
 	struct nexthop *nhp = nhge->nh_parent;
+	struct netlink_ext_ack extack;
 	struct nexthop *nh = nhge->nh;
 	struct nh_group *nhg, *newg;
-	int i, j;
+	int i, j, err;
 
 	WARN_ON(!nh);
 
@@ -942,6 +943,10 @@ static void remove_nh_grp_entry(struct net *net, struct nh_grp_entry *nhge,
 	list_del(&nhge->nh_list);
 	nexthop_put(nhge->nh);
 
+	err = call_nexthop_notifiers(net, NEXTHOP_EVENT_REPLACE, nhp, &extack);
+	if (err)
+		pr_err("%s\n", extack._msg);
+
 	if (nlinfo)
 		nexthop_notify(RTM_NEWNEXTHOP, nhp, nlinfo);
 }