xen: events: do not free legacy IRQs

c514d00c8057 "xen: events: add xen_allocate_irq_{dynamic, gsi} and
xen_free_irq" correctly avoids reallocating legacy IRQs (which are
managed by the arch core) but erroneously did not prevent them being
freed.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/events.c

@@ -434,6 +434,10 @@ static int xen_allocate_irq_gsi(unsigned gsi)
 
 static void xen_free_irq(unsigned irq)
 {
+	/* Legacy IRQ descriptors are managed by the arch. */
+	if (irq < NR_IRQS_LEGACY)
+		return;
+
 	irq_free_desc(irq);
 }