LU-6174 obd: perform proper division

LU-6174 obd: perform proper division

Lustre stats have two files lc_sum and lc_count which are both
s64 so using do_div() is completely wrong. Use div64_s64()
instead.

Change-Id: Ie694c1c6bf79979bff3eae0de9791c81c355ea30
Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-on: https://review.whamcloud.com/36751
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Shaun Tancheff <stancheff@cray.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>