LU-12585 obdfilter: Use actual I/O bytes in stats

LU-12585 obdfilter: Use actual I/O bytes in stats

Currently the obdfilter stats note the number of bytes
requested by the client rather than the number of bytes
actually read or written.  This is particularly confusing
for reads because clients can request more data than
exists and some applications do this normally.

This results in statistics that can be off by almost any
amount from the actual number of bytes read.  This patch
moves the stats to be collected just before commit, which
allows the true number of bytes to be recorded but does not
include the commit time in the time stats.  (Since commit
time is not part of the operation latency as experienced by
the client.)

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I81fe9a6afdad5b48e8421f4aa72b8ef10a0eee93
Reviewed-on: https://review.whamcloud.com/46075
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Aurelien Degremont <degremoa@amazon.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>