x86/MCE/AMD: Always give panic severity for UC errors in kernel context

The AMD severity grading function was introduced in kernel 4.1. The
current logic can possibly give MCE_AR_SEVERITY for uncorrectable
errors in kernel context. The system may then get stuck in a loop as
memory_failure() will try to handle the bad kernel memory and find it
busy.

Return MCE_PANIC_SEVERITY for all UC errors IN_KERNEL context on AMD
systems.

After:

  b2f9d678

 ("x86/mce: Check for faults tagged in EXTABLE_CLASS_FAULT exception table entries")

was accepted in v4.6, this issue was masked because of the tail-end attempt
at kernel mode recovery in the #MC handler.

However, uncorrectable errors IN_KERNEL context should always be considered
unrecoverable and cause a panic.

Signed-off-by: Yazen Ghannam <yazen.ghannam@amd.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: <stable@vger.kernel.org> # 4.9.x
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tony Luck <tony.luck@intel.com>
Cc: linux-edac <linux-edac@vger.kernel.org>
Fixes: bf80bbd7

 (x86/mce: Add an AMD severities-grading function)
Link: http://lkml.kernel.org/r/20171106174633.13576-1-bp@alien8.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>