library: correct 'use' calculations for <slabinfo> api

library: correct 'use' calculations for <slabinfo> api

Awhile back, the calculation for cache utilization was
corrected to avoid a 32 bit overflow. This commit just
brings this branch into line with that earlier change.

[ references shown below represent the master branch ]

Reference(s):
commit 23ba442c886f6250d1068a82fb7d0fc544acfd63
https://gitlab.com/procps-ng/procps/merge_requests/29
http://www.freelists.org/post/procps/fix-regression-created-by-99d71ad

Signed-off-by: Jim Warner <james.warner@comcast.net>