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>