nexthop: Emit a notification when a single nexthop is replaced

The notification is emitted after all the validation checks were
performed, but before the new configuration (i.e., 'struct nh_info') is
pointed at by the old shell (i.e., 'struct nexthop'). This prevents the
need to perform rollback in case the notification is vetoed.

The next patch will also emit a replace notification for all the nexthop
groups in which the nexthop is used.
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/nexthop.c

@@ -1100,12 +1100,22 @@ static int replace_nexthop_single(struct net *net, struct nexthop *old,
 				  struct netlink_ext_ack *extack)
 {
 	struct nh_info *oldi, *newi;
+	int err;
 
 	if (new->is_group) {
 		NL_SET_ERR_MSG(extack, "Can not replace a nexthop with a nexthop group.");
 		return -EINVAL;
 	}
 
+	err = call_nexthop_notifiers(net, NEXTHOP_EVENT_REPLACE, new, extack);
+	if (err)
+		return err;
+
+	/* Hardware flags were set on 'old' as 'new' is not in the red-black
+	 * tree. Therefore, inherit the flags from 'old' to 'new'.
+	 */
+	new->nh_flags |= old->nh_flags & (RTNH_F_OFFLOAD | RTNH_F_TRAP);
+
 	oldi = rtnl_dereference(old->nh_info);
 	newi = rtnl_dereference(new->nh_info);