pktgen: Fix position of ip and udp header

skb_set_network_header() expects an offset based on the data pointer
whereas skb_tail_offset() also includes the headroom. This resulted
in the ip header being written in a wrong location.

Use return values of skb_put() directly and rely on skb->len to
set mac, network, and transport header.

Cc: Simon Horman <horms@verge.net.au>
Cc: Daniel Borkmann <dborkmann@redhat.com>
Assisted-by: Daniel Borkmann <dborkmann@redhat.com>
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Acked-by: Daniel Borkmann <dborkmann@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>