iommu: Initialize domain->handler in iommu_domain_alloc()

Since it is not guaranteed that an iommu driver initializes in its
domain_init() function, it must be initialized with NULL to prevent
calling a function in an arbitrary location when iommu fault occurred.

Signed-off-by: KyongHo Cho <[e-mail]>
Signed-off-by: Joerg Roedel <[e-mail]>

drivers/iommu/iommu.c

···
 	if (bus == NULL || bus->iommu_ops == NULL)
 		return NULL;
 
-	domain = kmalloc(sizeof(*domain), GFP_KERNEL);
+	domain = kzalloc(sizeof(*domain), GFP_KERNEL);
 	if (!domain)
 		return NULL;