genirq: generic chip: Use DIV_ROUND_UP to calculate numchips

The number of interrupts in a domain may be not divisible by the
number of interrupts each chip handles. Integer division may truncate
the result, thus use DIV_ROUND_UP to count numchips.

Seems all users of irq_alloc_domain_generic_chips() in current code do
not have this issue. I just found the issue while reading the code.
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Cc: Grant Likely <grant.likely@linaro.org>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Link: http://lkml.kernel.org/r/1373015592.18252.2.camel@phoenixSigned-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/irq/generic-chip.c

@@ -278,7 +278,7 @@ int irq_alloc_domain_generic_chips(struct irq_domain *d, int irqs_per_chip,
 	if (d->revmap_type != IRQ_DOMAIN_MAP_LINEAR)
 		return -EINVAL;
 
-	numchips = d->revmap_data.linear.size / irqs_per_chip;
+	numchips = DIV_ROUND_UP(d->revmap_data.linear.size, irqs_per_chip);
 	if (!numchips)
 		return -EINVAL;