powerpc/xive: Export XIVE IPI information for online-only processors.

Cédric pointed out that XIVE IPI information exported via sysfs
(debug/powerpc/xive) display empty lines for processors which are
not online.

Switch to using for_each_online_cpu() so that information is
displayed for online-only processors.

Reported-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Sachin Sant <sachinp@linux.ibm.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/164146703333.19039.10920919226094771665.sendpatchset@MacBook-Pro.local

arch/powerpc/sysdev/xive/common.c

@@ -1791,7 +1791,7 @@ static int xive_ipi_debug_show(struct seq_file *m, void *private)
 	if (xive_ops->debug_show)
 		xive_ops->debug_show(m, private);
 
-	for_each_possible_cpu(cpu)
+	for_each_online_cpu(cpu)
 		xive_debug_show_ipi(m, cpu);
 	return 0;
 }