x86, UV: Fix NMI handler for UV platforms

This fixes problems seen on UV systems handling NMIs from the
node controller.

I isolated the "dazed..." messages that I saw earlier to a bug in
the BMC on our platform. It was sending NMIs w/o properly setting
a register that indicated the source of NMI.

So rather than _assuming_ any unhandled NMI came from the UV system
maintenance console (SMC), add a check to verify that the SMC actually
sent the NMI.

Signed-off-by: Jack Steiner <steiner@sgi.com>
Cc: gorcunov@gmail.com
Cc: dzickus@redhat.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>