x86, NMI: Add to_cpumask() to silence compile warning

Gcc complains if we don't cast this to a struct cpumask pointer.
arch/x86/kernel/nmi_selftest.c:93:2:

	warning: passing argument 1 of ‘cpumask_empty’ from
	incompatible pointer type [enabled by default]
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Don Zickus <dzickus@redhat.com>
Link: http://lkml.kernel.org/r/20111207110612.GA3437@mwandaSigned-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/nmi_selftest.c

@@ -90,7 +90,7 @@ static void remote_ipi(void)
 {
 	cpumask_copy(to_cpumask(nmi_ipi_mask), cpu_online_mask);
 	cpumask_clear_cpu(smp_processor_id(), to_cpumask(nmi_ipi_mask));
-	if (!cpumask_empty(nmi_ipi_mask))
+	if (!cpumask_empty(to_cpumask(nmi_ipi_mask)))
 		test_nmi_ipi(to_cpumask(nmi_ipi_mask));
 }