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

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>
Cc: <stable@vger.kernel.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>