pktgen: fix skb leak in case of failure

Seems that skb goes into void unless something magic happened
in pskb_expand_head in case of failure.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Acked-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>