ppc/xive2: Process group backlog when updating the CPPR

ppc/xive2: Process group backlog when updating the CPPR

When the hypervisor or OS pushes a new value to the CPPR, if the LSMFB
value is lower than the new CPPR value, there could be a pending group
interrupt in the backlog, so it needs to be scanned.

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>