ppc/xive2: Process group backlog when pushing an OS context

ppc/xive2: Process group backlog when pushing an OS context

When pushing an OS context, we were already checking if there was a
pending interrupt in the IPB and sending a notification if needed.  We
also need to check if there is a pending group interrupt stored in the
NVG table. To avoid useless backlog scans, we only scan if the NVP
belongs to a group.

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>