x86: fix probe_nr_irqs for xen

otherwise Xen is _completely_ unusable with 5 or more VCPUs.
(when !CONFIG_HAVE_SPARSE_IRQ).

based on Alex Nixon's patch.

also add +1 offset after redir_entries
Signed-off-by: Yinghai Lu <yhlu.kernel@gmail.com>
Acked-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Acked-by: Alex Nixon <alex.nixon@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/io_apic.c

@@ -3625,16 +3625,21 @@ int __init probe_nr_irqs(void)
 {
 	int idx;
 	int nr = 0;
+#ifndef CONFIG_XEN
+	int nr_min = 32;
+#else
+	int nr_min = NR_IRQS;
+#endif
 
 	for (idx = 0; idx < nr_ioapics; idx++)
-		nr += io_apic_get_redir_entries(idx);
+		nr += io_apic_get_redir_entries(idx) + 1;
 
 	/* double it for hotplug and msi and nmi */
 	nr <<= 1;
 
 	/* something wrong ? */
-	if (nr < 32)
-		nr = 32;
+	if (nr < nr_min)
+		nr = nr_min;
 
 	return nr;
 }