nexthop: Emit a notification when a nexthop group is replaced

Emit a notification in the nexthop notification chain when an existing
nexthop group is replaced.

The notification is emitted after all the validation checks were
performed, but before the new configuration (i.e., 'struct nh_grp') is
pointed at by the old shell (i.e., 'struct nexthop'). This prevents the
need to perform rollback in case the notification is vetoed.
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/nexthop.c

@@ -1050,13 +1050,17 @@ static int replace_nexthop_grp(struct net *net, struct nexthop *old,
 			       struct netlink_ext_ack *extack)
 {
 	struct nh_group *oldg, *newg;
-	int i;
+	int i, err;
 
 	if (!new->is_group) {
 		NL_SET_ERR_MSG(extack, "Can not replace a nexthop group with a nexthop.");
 		return -EINVAL;
 	}
 
+	err = call_nexthop_notifiers(net, NEXTHOP_EVENT_REPLACE, new, extack);
+	if (err)
+		return err;
+
 	oldg = rtnl_dereference(old->nh_grp);
 	newg = rtnl_dereference(new->nh_grp);