kaiser: x86: Fix NMI handling

kaiser: x86: Fix NMI handling

On Mon, 4 Dec 2017, Hugh Dickins wrote:

> kaiser-3.18.72.tar

Hi Hugh,

this hunk from 0024-kaiser-merged-update.patch:

	-       SWITCH_KERNEL_CR3_NO_STACK
	+       /*
	+        * percpu variables are mapped with user CR3, so no need
	+        * to switch CR3 here.
	+        */
	        cld
	        movq    %rsp, %rdx
	        movq    PER_CPU_VAR(kernel_stack), %rsp

is problematic, as the patchset actually never user-maps kernel_stack
percpu variable, and therefore crashes on NMIs.

The patch below is needed to make NMIs work properly.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>