genirq: Make irq_shutdown() symmetric vs. irq_startup again

If an irq_chip provides .irq_shutdown(), but neither of .irq_disable() or
.irq_mask(), free_irq() crashes when jumping to NULL.
Fix this by only trying .irq_disable() and .irq_mask() if there's no
.irq_shutdown() provided.

This revives the symmetry with irq_startup(), which tries .irq_startup(),
.irq_enable(), and irq_unmask(), and makes it consistent with the comment for
irq_chip.irq_shutdown() in <linux/irq.h>, which says:

* @irq_shutdown: shut down the interrupt (defaults to ->disable if NULL)

This is also how __free_irq() behaved before the big overhaul, cfr. e.g.
3b56f0585fd4c02d047dc406668cb40159b2d340 ("genirq: Remove bogus conditional"),
where the core interrupt code always overrode .irq_shutdown() to
.irq_disable() if .irq_shutdown() was NULL.

Signed-off-by: Geert Uytterhoeven <[e-mail]>
Cc: [e-mail]
Link: http://lkml.kernel.org/r/[e-mail]
Cc: [e-mail]
Signed-off-by: Thomas Gleixner <[e-mail]>

kernel/irq/chip.c

···
 	desc->depth = 1;
 	if (desc->irq_data.chip->irq_shutdown)
 		desc->irq_data.chip->irq_shutdown(&desc->irq_data);
-	if (desc->irq_data.chip->irq_disable)
+	else if (desc->irq_data.chip->irq_disable)
 		desc->irq_data.chip->irq_disable(&desc->irq_data);
 	else
 		desc->irq_data.chip->irq_mask(&desc->irq_data);