x86: fix off-by-one in nr_irqs_gsi calculation

highest_gsi() returns the last valid GSI, not a count.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

---
v2: Retain the (debatable) use of max() here.

--- a/xen/arch/x86/io_apic.c
+++ b/xen/arch/x86/io_apic.c
@@ -2531,7 +2531,7 @@ void __init init_ioapic_mappings(void)
         }
     }
 
-    nr_irqs_gsi = max(nr_irqs_gsi, highest_gsi());
+    nr_irqs_gsi = max(nr_irqs_gsi, highest_gsi() + 1);
 
     if ( max_gsi_irqs == 0 )
         max_gsi_irqs = nr_irqs ? nr_irqs / 8 : PAGE_SIZE;