net/tls: avoid NULL pointer deref on nskb->sk in fallback

update_chksum() accesses nskb->sk before it has been set
by complete_skb(), move the init up.

Fixes: e8f69799 ("net/tls: Add generic NIC offload infrastructure")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/tls/tls_device_fallback.c

@@ -201,13 +201,14 @@ static void complete_skb(struct sk_buff *nskb, struct sk_buff *skb, int headln)
 
 	skb_put(nskb, skb->len);
 	memcpy(nskb->data, skb->data, headln);
-	update_chksum(nskb, headln);
 
 	nskb->destructor = skb->destructor;
 	nskb->sk = sk;
 	skb->destructor = NULL;
 	skb->sk = NULL;
 
+	update_chksum(nskb, headln);
+
 	delta = nskb->truesize - skb->truesize;
 	if (likely(delta < 0))
 		WARN_ON_ONCE(refcount_sub_and_test(-delta, &sk->sk_wmem_alloc));