Kernel 4.15 lost set_robust_list support on POWER 9

[Florian Weimer]

I get this:

7041  set_robust_list(0x7fff93dc3980, 24) = -1 ENOSYS (Function not 
implemented)

The system call works on 4.14.  Looks like the probing for 
futex_cmpxchg_enabled goes wrong.

Sorry, I have no idea where to start digging.  I don't see anything 
obvious in dmesg.

I'm trying to revert

commit 30d6e0a4190d37740e9447e4e4815f06992dd8c3
Author: Jiri Slaby <jslaby at suse.cz>
Date:   Thu Aug 24 09:31:05 2017 +0200

     futex: Remove duplicated code and fix undefined behaviour

to see if it makes a difference.

Thanks,
Florian