x86, NMI: Clean-up default_do_nmi()

Just re-arrange the code a bit to make it easier to follow what is
going on.  Basically un-negating the if-statement and swapping the code
inside the if-statement with code outside.

No functional changes.

Originally-by: Huang Ying <ying.huang@intel.com>
Signed-off-by: Don Zickus <dzickus@redhat.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1294348732-15030-7-git-send-email-dzickus@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>