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@mwanda
Signed-off-by: Ingo Molnar <mingo@elte.hu>