x86: Properly _init-annotate NMI selftest code

After all, this code is being run once at boot only (if
configured in at all).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Don Zickus <dzickus@redhat.com>
Link: http://lkml.kernel.org/r/4F478C010200007800074A3D@nat28.tlf.novell.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>