reopen #963689 and provide a better patch for using VMAP_STACK on powerpc

[Giuseppe Sacco <giuseppe@sguazz.it>]

reopen 963689
forwarded 963689 https://github.com/linuxppc/issues/issues/326
tags 963689 +patch
thanks

Hello, there is now a better patch for this problem. This one does not
disable VMAP_STACK but correctly handle the CPU state saving/restore on
sleep.

The patch is 
https://git.kernel.org/pub/scm/linux/kernel/git/powerpc/linux.git/commit/?id=db972a3787d12b1ce9ba7a31ec376d8a79e04c47
and has been just applied to powerpc/next.

Thank you,
Giuseppe