rxperf: print achieved transfer rate

Prints the client transfer rate in appropriate units (kbit/s, Mbit/s, or Gbit/s).

Change-Id: I5632185953c5d2bfc962af71152734d842cfb021
Reviewed-on: http://gerrit.openafs.org/8758
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Simon Wilkinson <simonxwilkinson@gmail.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>