[PATCH net-next-2.6] pktgen: bug fix in transmission headers with frags=0

[Daniel Turull <daniel.turull@gmail.com>]

(bug introduced by commit 26ad787962ef84677a48c560
(pktgen: speedup fragmented skbs)

The headers of pktgen were incorrectly added in a pktgen packet
without frags (frags=0). There was an offset in the pktgen headers.

The cause was in reusing the pgh variable as a return variable in skb_put
when adding the payload to the skb.

Signed-off-by: Daniel Turull <daniel.turull@gmail.com>

---
The PKTGEN magic (be9b e955) now starts in the correct offset.
Before the patch, it was starting with an offset of +16 bytes
(sizeof(struct pktgen_hdr)) (be9b)

Capture from tcpdump:

before patch:
14:57:37.854812 IP 10.0.0.2.discard > 10.254.254.84.discard: UDP, length 18
	0x0000:  001b 2157 ed84 001b 215d 01d0 0800 4500
	0x0010:  002e 0004 0000 2011 8767 0a00 0002 0afe
	0x0020:  fe54 0009 0009 001a 0000 0000 0000 b072
	0x0030:  9102 00ea ffff 0010 0000 be9b

after patch:
14:44:32.896048 IP 10.0.0.2.discard > 10.217.234.56.discard: UDP, length 18
	0x0000:  001b 2157 ed84 001b 215d 01d0 0800 4500
	0x0010:  002e 0000 0000 2011 9bac 0a00 0002 0ad9
	0x0020:  ea38 0009 0009 001a 0000 be9b e955 0000
	0x0030:  0001 4d7e 1b09 0005 5f23 af00

---
diff --git a/net/core/pktgen.c b/net/core/pktgen.c
index f0aec6c..0c55eaa 100644
--- a/net/core/pktgen.c
+++ b/net/core/pktgen.c
@@ -2620,8 +2620,7 @@ static void pktgen_finalize_skb(struct pktgen_dev *pkt_dev, struct sk_buff *skb,
 	datalen -= sizeof(*pgh);
 
 	if (pkt_dev->nfrags <= 0) {
-		pgh = (struct pktgen_hdr *)skb_put(skb, datalen);
-		memset(pgh + 1, 0, datalen);
+		memset(skb_put(skb, datalen), 0, datalen);
 	} else {
 		int frags = pkt_dev->nfrags;
 		int i, len;