LU-9887 lfsck: calculate LFSCK speed properly

LU-9887 lfsck: calculate LFSCK speed properly

Originally, we used do_div(a,b) to calculate the LFSCK average
speed, and got the result from the parameter @a. But later, we
replaced do_div(a,b) with div_u64(a,b). The latter one doesn't
stores the quotient in the parameter @a, instead, the quotient
is returned via the function return value. The patch fixes the
LFSCK logic to obtain the LFSCK average speed from div_u64(a,b)
return value.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I442fb8f7e6c51a4853ea37694e3c221f97e26b19
Reviewed-on: https://review.whamcloud.com/28617
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>