pktgen: bug fix in transmission headers with frags=0

(bug introduced by commit 26ad7879
(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>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>