gpiolib: Fix irq_disable() semantics

The implementation if .irq_disable() which kicks in between
the gpiolib and the driver is not properly mimicking the
expected semantics of the irqchip core: the irqchip will
call .irq_disable() if that exists, else it will call
mask_irq() which first checks if .irq_mask() is defined
before calling it.

Since we are calling it unconditionally, we get this bug
from drivers/pinctrl/qcom/pinctrl-ssbi-gpio.c, as it only
defines .irq_mask_ack and not .irq_mask:

  Unable to handle kernel NULL pointer dereference at virtual address 00000000
  pgd = (ptrval)
  (...)
  PC is at 0x0
  LR is at gpiochip_irq_disable+0x20/0x30

Fix this by only calling .irq_mask() if it exists.

Cc: Brian Masney <masneyb@onstation.org>
Cc: Hans Verkuil <hans.verkuil@cisco.com>
Cc: stable@vger.kernel.org
Reviewed-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Fixes: 461c1a7d ("gpiolib: override irq_enable/disable")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20200306132326.1329640-1-linus.walleij@linaro.org

drivers/gpio/gpiolib.c

@@ -2306,9 +2306,16 @@ static void gpiochip_irq_disable(struct irq_data *d)
 {
 	struct gpio_chip *chip = irq_data_get_irq_chip_data(d);
 
+	/*
+	 * Since we override .irq_disable() we need to mimic the
+	 * behaviour of __irq_disable() in irq/chip.c.
+	 * First call .irq_disable() if it exists, else mimic the
+	 * behaviour of mask_irq() which calls .irq_mask() if
+	 * it exists.
+	 */
 	if (chip->irq.irq_disable)
 		chip->irq.irq_disable(d);
-	else
+	else if (chip->irq.chip->irq_mask)
 		chip->irq.chip->irq_mask(d);
 	gpiochip_disable_irq(chip, d->hwirq);
 }