inet: introduce dst_rtable() helper

I added dst_rt6_info() in commit
e8dfd42c17 ("ipv6: introduce dst_rt6_info() helper")

This patch does a similar change for IPv4.

Instead of (struct rtable *)dst casts, we can use :

 #define dst_rtable(_ptr) \
             container_of_const(_ptr, struct rtable, dst)

Patch is smaller than IPv6 one, because IPv4 has skb_rtable() helper.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://lore.kernel.org/r/20240429133009.1227754-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/ip_input.c

@@ -616,7 +616,7 @@ static void ip_list_rcv_finish(struct net *net, struct sock *sk,
 		dst = skb_dst(skb);
 		if (curr_dst != dst) {
 			hint = ip_extract_route_hint(net, skb,
-					       ((struct rtable *)dst)->rt_type);
+						     dst_rtable(dst)->rt_type);
 
 			/* dispatch old sublist */
 			if (!list_empty(&sublist))