ipv4: mentions skb_gro_postpull_rcsum() in inet_gro_receive()

Proper CHECKSUM_COMPLETE support needs to adjust skb->csum
when we remove one header. Its done using skb_gro_postpull_rcsum()

In the case of IPv4, we know that the adjustment is not really needed,
because the checksum over IPv4 header is 0. Lets add a comment to
ease code comprehension and avoid copy/paste errors.
Signed-off-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/af_inet.c

@@ -1369,6 +1369,9 @@ static struct sk_buff **inet_gro_receive(struct sk_buff **head,
 	 * immediately following this IP hdr.
 	 */
 
+	/* Note : No need to call skb_gro_postpull_rcsum() here,
+	 * as we already checked checksum over ipv4 header was 0
+	 */
 	skb_gro_pull(skb, sizeof(*iph));
 	skb_set_transport_header(skb, skb_gro_offset(skb));