genirq: Use cpumask_available() for check of cpumask variable

This fixes the following clang warning when CONFIG_CPUMASK_OFFSTACK=n:

kernel/irq/manage.c:839:28: error: address of array
'desc->irq_common_data.affinity' will always evaluate to 'true'
[-Werror,-Wpointer-bool-conversion]

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Cc: Grant Grundler <grundler@chromium.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Greg Hackmann <ghackmann@google.com>
Cc: Michael Davidson <md@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20170412182030.83657-2-mka@chromium.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/irq/manage.c

@@ -852,7 +852,7 @@ irq_thread_check_affinity(struct irq_desc *desc, struct irqaction *action)
 	 * This code is triggered unconditionally. Check the affinity
 	 * mask pointer. For CPU_MASK_OFFSTACK=n this is optimized out.
 	 */
-	if (desc->irq_common_data.affinity)
+	if (cpumask_available(desc->irq_common_data.affinity))
 		cpumask_copy(mask, desc->irq_common_data.affinity);
 	else
 		valid = false;