LU-7635 utils: Fix lhsmtool_posix interval reporting

LU-7635 utils: Fix lhsmtool_posix interval reporting

At specified time intervals lhsmtool_posix reports how much data it's
written.  It should report how much data has been written since last
update, but it reports total data written.

Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: I0e85b81fa2a8cf16474cc832bca30bf1425fa81c
Reviewed-on: http://review.whamcloud.com/17878
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Robert Read <robert.read@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>