net: Fix skb_copy_expand() handling of ->csum_start

It should only be adjusted if ip_summed == CHECKSUM_PARTIAL.
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -932,7 +932,8 @@ struct sk_buff *skb_copy_expand(const struct sk_buff *skb,
 	copy_skb_header(n, skb);
 
 	off                  = newheadroom - oldheadroom;
-	n->csum_start       += off;
+	if (n->ip_summed == CHECKSUM_PARTIAL)
+		n->csum_start += off;
 #ifdef NET_SKBUFF_DATA_USES_OFFSET
 	n->transport_header += off;
 	n->network_header   += off;