ppc/xive2: Add undelivered group interrupt to backlog

ppc/xive2: Add undelivered group interrupt to backlog

When a group interrupt cannot be delivered, we need to:
- increment the backlog counter for the group in the NVG table
  (if the END is configured to keep a backlog).
- start a broadcast operation to set the LSMFB field on matching CPUs
  which can't take the interrupt now because they're running at too
  high a priority.

[npiggin: squash in fixes from milesg]
[milesg: only load the NVP if the END is !ignore]
[milesg: always broadcast backlog, not only when there are precluded VPs]

Signed-off-by: Frederic Barrat <fbarrat@linux.ibm.com>
Signed-off-by: Michael Kowal <kowal@linux.ibm.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>