net: Fix useless comment reference loop.

include/linux/skbuff.h says:
        /* These elements must be at the end, see alloc_skb() for details.  */

net/core/skbuff.c says:
	* See comment in sk_buff definition, just before the 'tail' member

This patch contains my guess as to the actual reason rather than a
dead comment reference loop.
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/skbuff.c

@@ -200,7 +200,9 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
 		goto nodata;
 
 	/*
-	 * See comment in sk_buff definition, just before the 'tail' member
+	 * Only clear those fields we need to clear, not those that we will
+	 * actually initialise below. Hence, don't put any more fields after
+	 * the tail pointer in struct sk_buff!
 	 */
 	memset(skb, 0, offsetof(struct sk_buff, tail));
 	skb->truesize = size + sizeof(struct sk_buff);