ipv4: use sk_fullsock() in ipv4_conntrack_defrag()

Before converting a 'socket pointer' into inet socket,
use sk_fullsock() to detect timewait or request sockets.

Fixes: ca6fb06518 ("tcp: attach SYNACK messages to request sockets instead of listener")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Tested-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/netfilter/nf_defrag_ipv4.c

@@ -67,10 +67,9 @@ static unsigned int ipv4_conntrack_defrag(void *priv,
 					  const struct nf_hook_state *state)
 {
 	struct sock *sk = skb->sk;
-	struct inet_sock *inet = inet_sk(skb->sk);
 
-	if (sk && (sk->sk_family == PF_INET) &&
+	if (sk && sk_fullsock(sk) && (sk->sk_family == PF_INET) &&
-	    inet->nodefrag)
+	    inet_sk(sk)->nodefrag)
 		return NF_ACCEPT;
 
 #if IS_ENABLED(CONFIG_NF_CONNTRACK)