pktgen: Fix position of ip and udp header

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 <[email protected]>
Cc: Daniel Borkmann <[email protected]>
Assisted-by: Daniel Borkmann <[email protected]>
Signed-off-by: Thomas Graf <[email protected]>
Acked-by: Daniel Borkmann <[email protected]>
Signed-off-by: David S. Miller <[email protected]>