net: ipv4: Fix NULL pointer dereference in route lookup

When calculating the multipath hash for input routes the flow info is
not available and therefore should not be used.

Fixes: 24ba144 ("route: Add multipath_hash in flowi_common to make user-define hash")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Cc: wenxu <wenxu@ucloud.cn>
Acked-by: wenxu <wenxu@ucloud.cn>
Acked-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/route.c

@@ -1820,7 +1820,7 @@ static void ip_multipath_l3_keys(const struct sk_buff *skb,
 int fib_multipath_hash(const struct net *net, const struct flowi4 *fl4,
 		       const struct sk_buff *skb, struct flow_keys *flkeys)
 {
-	u32 multipath_hash = fl4->flowi4_multipath_hash;
+	u32 multipath_hash = fl4 ? fl4->flowi4_multipath_hash : 0;
 	struct flow_keys hash_keys;
 	u32 mhash;