Timing performance regression 4.15 to 4.16-rc1

[Zdenek Kabelac]

Hi

I've noticed quite some drop of performance (around 15% in some cases) where 
execution of lvm2 tests took longer time - and while tests itself should not 
really load CPU system  - the overall running time just got bigger.

Running bisect game pointed clearly to this commit:

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commit 44c02a2c3dc55835e9f0d8ef73966406cd805001
Author: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Date:   Thu Oct 5 12:59:44 2017 -0400

    dev_ioctl(): move copyin/copyout to callers


---

clear revert of this commit on top of 3266b5bd97eaa72793df0b6e5a106c69ccc166c4
(recent  ~4.16-rc4)  restored timing of tests back.


I'm not sure why - so at this moment this is just a patch causing 15% longer 
running times on lvm2 test suite.


Regards

Zdenek