powerpc/smp: Remove migrate_irq() custom implementation

Some powerpc platforms use this to move IRQs away from a CPU being
unplugged. This function has several bugs such as not taking the right
locks or failing to NULL check pointers.

There's a new generic function doing exactly the same thing without all
the bugs, so let's use it instead.

mpe: The obvious place for the select of GENERIC_IRQ_MIGRATION is on
HOTPLUG_CPU, but that doesn't work. On some configs PM_SLEEP_SMP will
select HOTPLUG_CPU even though its dependencies are not met, which means
the select of GENERIC_IRQ_MIGRATION doesn't happen. That leads to the
build breaking. Fix it by moving the select of GENERIC_IRQ_MIGRATION to
SMP.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/smp.c

@@ -439,7 +439,14 @@ int generic_cpu_disable(void)
 #ifdef CONFIG_PPC64
 	vdso_data->processorCount--;
 #endif
-	migrate_irqs();
+	/* Update affinity of all IRQs previously aimed at this CPU */
+	irq_migrate_all_off_this_cpu();
+
+	/* Give the CPU time to drain in-flight ones */
+	local_irq_enable();
+	mdelay(1);
+	local_irq_disable();
+
 	return 0;
 }