irq/irq_domain: Quit ignoring error returns from irq_alloc_desc_from().

In commit 4bbdd45 (irq_domain/powerpc: eliminate irq_map; use
irq_alloc_desc() instead) code was added that ignores error returns
from irq_alloc_desc_from() by (silently) casting the return value to
unsigned.  The negitive value error return now suddenly looks like a
valid irq number.

Commits cc79ca6 (irq_domain: Move irq_domain code from powerpc to
kernel/irq) and 1bc04f2 (irq_domain: Add support for base irq and
hwirq in legacy mappings) move this code to its current location in
irqdomain.c

The result of all of this is a null pointer dereference OOPS if one of
the error cases is hit.

The fix: Don't cast away the negativeness of the return value and then
check for errors.

Signed-off-by: David Daney <david.daney@cavium.com>
Acked-by: Rob Herring <rob.herring@calxeda.com>
[grant.likely: dropped addition of new 'irq' variable]
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

kernel/irq/irqdomain.c

@@ -350,7 +350,8 @@ unsigned int irq_create_direct_mapping(struct irq_domain *domain)
 unsigned int irq_create_mapping(struct irq_domain *domain,
 				irq_hw_number_t hwirq)
 {
-	unsigned int virq, hint;
+	unsigned int hint;
+	int virq;
 
 	pr_debug("irq: irq_create_mapping(0x%p, 0x%lx)\n", domain, hwirq);
 
@@ -381,9 +382,9 @@ unsigned int irq_create_mapping(struct irq_domain *domain,
 	if (hint == 0)
 		hint++;
 	virq = irq_alloc_desc_from(hint, 0);
-	if (!virq)
+	if (virq <= 0)
 		virq = irq_alloc_desc_from(1, 0);
-	if (!virq) {
+	if (virq <= 0) {
 		pr_debug("irq: -> virq allocation failed\n");
 		return 0;
 	}