powerpc/ibmebus: Get rid of the IRQ mapping in ibmebus_free_irq()

ibmebus_free_irq() frees the IRQ but does not remove its mapping, which
results in stale entries in the map.

This fixes it by adding a call to irq_dispose_mapping() in
ibmebus_free_irq().
Signed-off-by: Sebastien Dugue <sebastien.dugue@bull.net>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kernel/ibmebus.c

@@ -231,6 +231,7 @@ void ibmebus_free_irq(u32 ist, void *dev_id)
 	unsigned int irq = irq_find_mapping(NULL, ist);
 
 	free_irq(irq, dev_id);
+	irq_dispose_mapping(irq);
 }
 EXPORT_SYMBOL(ibmebus_free_irq);