ip: Fix a data-race around sysctl_fwmark_reflect.

While reading sysctl_fwmark_reflect, it can be changed concurrently.
Thus, we need to add READ_ONCE() to its reader.

Fixes: e110861f ("net: add a sysctl to reflect the fwmark on replies")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/ip.h

@@ -384,7 +384,7 @@ void ipfrag_init(void);
 void ip_static_sysctl_init(void);
 
 #define IP4_REPLY_MARK(net, mark) \
-	((net)->ipv4.sysctl_fwmark_reflect ? (mark) : 0)
+	(READ_ONCE((net)->ipv4.sysctl_fwmark_reflect) ? (mark) : 0)
 
 static inline bool ip_is_fragment(const struct iphdr *iph)
 {