CPUs ignoring NMIs is often a sign of those CPUs going bad, but there
are quite a few other reasons why a CPU might ignore NMIs.  Therefore,
accumulate evidence within exc_nmi() as to what might be preventing a
given CPU from responding to an NMI.

[ paulmck: Apply Peter Zijlstra feedback. ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: <x86@kernel.org>
Reviewed-by: Ingo Molnar <mingo@kernel.org>