nh: fix scope used to find saddr when adding non gw nh

As explained by Julian, fib_nh_scope is related to fib_nh_gw4, but
fib_info_update_nhc_saddr() needs the scope of the route, which is
the scope "before" fib_nh_scope, ie fib_nh_scope - 1.

This patch fixes the problem described in commit 747c1430 ("ip: fix
dflt addr selection for connected nexthop").

Fixes: 597cfe4f ("nexthop: Add support for IPv4 nexthops")
Link: https://lore.kernel.org/netdev/6c8a44ba-c2d5-cdf-c5c7-5baf97cba38@ssi.bg/Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Reviewed-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/nexthop.c

@@ -2534,7 +2534,7 @@ static int nh_create_ipv4(struct net *net, struct nexthop *nh,
 	if (!err) {
 		nh->nh_flags = fib_nh->fib_nh_flags;
 		fib_info_update_nhc_saddr(net, &fib_nh->nh_common,
-					  fib_nh->fib_nh_scope);
+					  !fib_nh->fib_nh_scope ? 0 : fib_nh->fib_nh_scope - 1);
 	} else {
 		fib_nh_release(net, fib_nh);
 	}