tcp: Do not reload skb pointer after skb_gro_receive().

This is not necessary.  skb_gro_receive() will never change what
'head' points to.

In it's original implementation (see commit 71d93b39 ("net: Add
skb_gro_receive")), it did:

====================
+	*head = nskb;
+	nskb->next = p->next;
+	p->next = NULL;
====================

This sequence was removed in commit 58025e46 ("net: gro: remove
obsolete code from skb_gro_receive()")
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Eric Dumazet <edumazet@google.com>

net/ipv4/tcp_offload.c

@@ -268,8 +268,6 @@ struct sk_buff **tcp_gro_receive(struct sk_buff **head, struct sk_buff *skb)
 		goto out_check_final;
 	}
 
-	p = *head;
-	th2 = tcp_hdr(p);
 	tcp_flag_word(th2) |= flags & (TCP_FLAG_FIN | TCP_FLAG_PSH);
 
 out_check_final: