x86/mce: Fix check for processor context when machine check was taken.

x86/mce: Fix check for processor context when machine check was taken.

commit 875e26648cf9b6db9d8dc07b7959d7c61fb3f49c upstream.

Linus pointed out that there was no value is checking whether m->ip
was zero - because zero is a legimate value.  If we have a reliable
(or faked in the VM86 case) "m->cs" we can use it to tell whether we
were in user mode or kernelwhen the machine check hit.

Reported-by: Linus Torvalds <torvalds@linuxfoundation.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>