tcp: fix a potential NULL pointer dereference in tcp_sk_exit

 When tcp_sk_init() failed in inet_ctl_sock_create(),
 'net->ipv4.tcp_congestion_control' will be left
 uninitialized, but tcp_sk_exit() hasn't check for
 that.

 This patch add checking on 'net->ipv4.tcp_congestion_control'
 in tcp_sk_exit() to prevent NULL-ptr dereference.

Fixes: 6670e152 ("tcp: Namespace-ify sysctl_tcp_default_congestion_control")
Signed-off-by: Dust Li <dust.li@linux.alibaba.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp_ipv4.c

@@ -2578,7 +2578,8 @@ static void __net_exit tcp_sk_exit(struct net *net)
 {
 	int cpu;
 
-	module_put(net->ipv4.tcp_congestion_control->owner);
+	if (net->ipv4.tcp_congestion_control)
+		module_put(net->ipv4.tcp_congestion_control->owner);
 
 	for_each_possible_cpu(cpu)
 		inet_ctl_sock_destroy(*per_cpu_ptr(net->ipv4.tcp_sk, cpu));