block/blk-iolatency: fix always-zero cur_stat mean on non-SSD

[blktests-ci]

Pull request for series with
subject: block/blk-iolatency: fix always-zero cur_stat mean on non-SSD
version: 3
url: https://patchwork.kernel.org/series/1157639/

[blktests-ci]

Upstream branch: ba285ed
series: https://patchwork.kernel.org/series/1157639/
version: 3

Pull request is NOT updated. Failed to apply https://patchwork.kernel.org/series/1157639/
error message:

Cmd('git') failed due to: exit code(128)
  cmdline: git am --3way
  stdout: 'Patch is empty.
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To record the empty patch as an empty commit, run "git am --allow-empty".
To restore the original branch and stop patching, run "git am --abort".'

conflict: