pktgen: Add output for imix results

The bps for imix mode is calculated by:
sum(imix_entry.size) / time_elapsed

The actual counts of each imix_entry are displayed under the
"Current:" section of the interface output in the following format:
imix_size_counts: size_1,count_1 size_2,count_2 ... size_n,count_n

Example (count = 200000):
imix_weights: 256,1 859,3 205,2
imix_size_counts: 256,32082 859,99796 205,68122
Result: OK: 17992362(c17964678+d27684) usec, 200000 (859byte,0frags)
  11115pps 47Mb/sec (47977140bps) errors: 0

Summary of changes:
Calculate bps based on imix counters when in IMIX mode.
Add output for IMIX counters.

Signed-off-by: Nick Richardson <richardsonnick@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>