trafgen: performance regression at 10G due to smaller ring buffer size

commit c10621e ("trafgen: keep a small initial ring buffer size to
reduce cache-misses") reduced default ring buffer size to 196KiB,
but on my big machines with 10Gbit/s this size is too small.

Increase default ring-size to 512 KiB, yield the best results,
without increasing ring buffer size too much, this fixes #120.

Single CPU results from my E5-2630 CPU with intel ixgbe/82599.
(Cmd: trafgen --cpp --dev eth8 --conf udp_example01.trafgen --cpu 1)

 * 769,440 pkts/sec -- default ring-size 196 KiB

 * 1,417,908 pkts/sec -- ring-size 500 KiB

Going above CPUs L3 cache size which is (15Mb)

 * 1,236,580 pkts/sec -- ring-size 20000KiB

The mmap'ed ring buffer is now faster than using sendto().
For comparison, not using the ring-buffer, by using option "-t0":

 * 1,381,364 (with qdisc bypass)

And using the qdisc code path in the kernel (enable via
parameter "--qdisc-path")

 * 1,227,772 pkts/sec (with --qdisc-path)

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>