os_mon: Fix disksup full threshold testcase
os_mon: Fix disksup full threshold testcase

On systems with 0% of used disk space this testcase used to
fail. Note that 0% does not mean that 9 bytes was used, just
that the df command rounded down to 0% usage.

potatosalad/otp,isvilen/otp,potatosalad/otp,bjorng/otp,mikpe/otp,vinoski/otp,erlang/otp,bjorng/otp,erlang/otp,mikpe/otp,g-andrade/otp,dgud/otp,dgud/otp,emacsmirror/erlang,mikpe/otp,g-andrade/otp,isvilen/otp,dgud/otp,erlang/otp,emacsmirror/erlang,mikpe/otp,g-andrade/otp,bjorng/otp,erlang/otp,g-andrade/otp,dgud/otp,bjorng/otp,vinoski/otp,erlang/otp,vinoski/otp,potatosalad/otp,isvilen/otp,erlang/otp,mikpe/otp,vinoski/otp,g-andrade/otp,dgud/otp,g-andrade/otp,potatosalad/otp,bjorng/otp,vinoski/otp,isvilen/otp,mikpe/otp,emacsmirror/erlang,isvilen/otp,bjorng/otp,emacsmirror/erlang,isvilen/otp,g-andrade/otp,erlang/otp,mikpe/otp,potatosalad/otp,g-andrade/otp,bjorng/otp,emacsmirror/erlang,erlang/otp,potatosalad/otp,dgud/otp,emacsmirror/erlang,vinoski/otp,erlang/otp,emacsmirror/erlang,dgud/otp,g-andrade/otp,bjorng/otp,potatosalad/otp,g-andrade/otp,vinoski/otp,erlang/otp,vinoski/otp,mikpe/otp,vinoski/otp,emacsmirror/erlang,bjorng/otp,mikpe/otp,potatosalad/otp,vinoski/otp,isvilen/otp,potatosalad/otp,isvilen/otp,dgud/otp,potatosalad/otp,emacsmirror/erlang,isvilen/otp,emacsmirror/erlang,bjorng/otp,dgud/otp,mikpe/otp,dgud/otp,isvilen/otp