net: add the comment for skb->l4_rxhash

Signed-off-by: Changli Gao <xiaosuo@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index f902c33..ea0b374 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -322,6 +322,8 @@
  *	@queue_mapping: Queue mapping for multiqueue devices
  *	@ndisc_nodetype: router type (from link layer)
  *	@ooo_okay: allow the mapping of a socket to a queue to be changed
+ *	@l4_rxhash: indicate rxhash is a canonical 4-tuple hash over transport
+ *		ports.
  *	@dma_cookie: a cookie to one of several possible DMA operations
  *		done by skb DMA functions
  *	@secmark: security marking