LU-2395 ofd: Maintain per-export {read,write}_bytes counters

LU-2395 ofd: Maintain per-export {read,write}_bytes counters

Add handling of these counters to ofd_counter_incr() and add a test to
sanity to ensure that they work as intended.

Signed-off-by: John L. Hammond <jhammond@tacc.utexas.edu>
Change-Id: I49e237a6387b8ece1aecd63987ad4a75897b430b
Reviewed-on: http://review.whamcloud.com/4687
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>