x86/platform: Remove warning message for duplicate NMI handlers

Remove the WARNING message associated with multiple NMI handlers as
there are at least two that are legitimate.  These are the KGDB and the
UV handlers and both want to be called if the NMI has not been claimed
by any other NMI handler.

Use of the UNKNOWN NMI call chain dramatically lowers the NMI call rate
when high frequency NMI tools are in use, notably the perf tools.  It is
required on systems that cannot sustain a high NMI call rate without
adversely affecting the system operation.

Signed-off-by: Mike Travis <mike.travis@hpe.com>
Reviewed-by: Dimitri Sivanich <dimitri.sivanich@hpe.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Russ Anderson <russ.anderson@hpe.com>
Cc: Frank Ramsay <frank.ramsay@hpe.com>
Cc: Tony Ernst <tony.ernst@hpe.com>
Link: http://lkml.kernel.org/r/20170307210841.730959611@asylum.americas.sgi.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/kernel/nmi.c

@@ -166,11 +166,9 @@ int __register_nmi_handler(unsigned int type, struct nmiaction *action)
 	spin_lock_irqsave(&desc->lock, flags);
 
 	/*
-	 * most handlers of type NMI_UNKNOWN never return because
-	 * they just assume the NMI is theirs.  Just a sanity check
-	 * to manage expectations
+	 * Indicate if there are multiple registrations on the
+	 * internal NMI handler call chains (SERR and IO_CHECK).
 	 */
-	WARN_ON_ONCE(type == NMI_UNKNOWN && !list_empty(&desc->head));
 	WARN_ON_ONCE(type == NMI_SERR && !list_empty(&desc->head));
 	WARN_ON_ONCE(type == NMI_IO_CHECK && !list_empty(&desc->head));