x86/apic: Remove noisy zero-mask warning from default_send_IPI_mask_logical()

Since circa 3.5, we've had dozens of reports of people hitting
this warning. Forwarded reports have been met with silence, so
just remove the warning if no-one cares.

Example reports:

  https://bugzilla.redhat.com/show_bug.cgi?id=797687
  https://bugzilla.redhat.com/show_bug.cgi?id=867174
  https://bugzilla.redhat.com/show_bug.cgi?id=894865

Signed-off-by: Dave Jones <davej@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20130118175847.GA27662@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

Author: Dave Jones

arch/x86/kernel/apic/ipi.c

@@ -106,7 +106,7 @@ void default_send_IPI_mask_logical(const struct cpumask *cpumask, int vector)
 	unsigned long mask = cpumask_bits(cpumask)[0];
 	unsigned long flags;
 
-	if (WARN_ONCE(!mask, "empty IPI mask"))
+	if (!mask)
 		return;
 
 	local_irq_save(flags);