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 after 'commit', which
allows the true number of bytes to be recorded.

It also adds the 'commit' operation to the time recorded in
the stats for each operation, which is reasonable because
the commit must occur before the operation is complete.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I81fe9a6afdad5b48e8421f4aa72b8ef10a0eee93