test: packet_gen: add lso support

Add support for transmitting packets with Large Send Offload (LSO). LSO
can be enabled with (-T, --lso) command line option. Initially supported
LSO protocols are ODP_LSO_PROTO_IPV4, ODP_LSO_PROTO_TCP_IPV4, and
ODP_LSO_PROTO_CUSTOM.

Signed-off-by: Matias Elo <[email protected]>
Reviewed-by: Tuomas Taipale <[email protected]>