iommu/iova: Optimize iova_magazine_alloc()

Only the member 'size' needs to be initialized to 0. Clearing the array
pfns[], which is about 1 KiB in size, not only wastes time, but also
causes cache pollution.
Acked-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Link: https://lore.kernel.org/r/20230421072422.869-1-thunder.leizhen@huawei.comSigned-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/iova.c

@@ -647,7 +647,13 @@ struct iova_rcache {
 
 static struct iova_magazine *iova_magazine_alloc(gfp_t flags)
 {
-	return kzalloc(sizeof(struct iova_magazine), flags);
+	struct iova_magazine *mag;
+
+	mag = kmalloc(sizeof(*mag), flags);
+	if (mag)
+		mag->size = 0;
+
+	return mag;
 }
 
 static void iova_magazine_free(struct iova_magazine *mag)