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>