Although sizeof is an operator in C. Preferred usage is to use
it as a function.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>

--- a/drivers/net/virtio_net.c	2016-07-26 09:13:16.173384238 -0700
+++ b/drivers/net/virtio_net.c	2016-07-26 09:13:16.169384220 -0700
@@ -272,7 +272,7 @@ static struct sk_buff *page_to_skb(struc
 
 	hdr_len = vi->hdr_len;
 	if (vi->mergeable_rx_bufs)
-		hdr_padded_len = sizeof *hdr;
+		hdr_padded_len = sizeof(struct virtio_net_hdr_mrg_rxbuf);
 	else
 		hdr_padded_len = sizeof(struct padded_vnet_hdr);