pktgen: Rework to improve performance using low CPU frequency

pktgen: Rework to improve performance using low CPU frequency

The performance improvement is measured on AMD970 operating at 800Mhz
w/ Intel 82575 NIC using 18bytes UDP datagrams.

- Don't refill and allocate new packets for each transmission, instead
  a set of pregenerated packets are used.
- Use NIC driver's "txeof" to tick out more packets instead of bindly
  trying to inject more packets into NIC's TX ring.

These above two modifications give me ~200Kpps (710Kpps -> 910Kpps)
transmission rate improvement.

- Don't use hardware TX checksum offloading feature.  Since the set of
  packets to be used are pregenerated, this will not hurt performance,
  instead, this could improve performance on certain hardwares, e.g.
  Intel's NIC chipsets which require additional TX descriptor for setting
  up hardware TX checksum offload context.

The above modification give me additional ~40Kpps transmission rate
improvement.