pktgen: xmit logic reorganization

Do some reorganization of transmit logic path:
   * move transmit queue full idle to separate routine
   * add a cpu_relax()
   * eliminate some of the uneeded goto's
   * if queue is still stopped, go back to main thread loop.
   * don't give up transmitting if quantum is exhausted (be greedy)

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>