iommu/io-pgtable-arm: dynamically allocate selftest device struct

In general a 'struct device' is way too large to be put on the kernel
stack. Apparently something just caused it to grow a slightly larger,
which pushed the arm_lpae_do_selftests() function over the warning
limit in some configurations:

drivers/iommu/io-pgtable-arm.c:1423:19: error: stack frame size (1032) exceeds limit (1024) in 'arm_lpae_do_selftests' [-Werror,-Wframe-larger-than]
 1423 | static int __init arm_lpae_do_selftests(void)
      |                   ^

Change the function to use a dynamically allocated faux_device
instead of the on-stack device structure.

Fixes: ca25ec2 ("iommu/io-pgtable-arm: Remove iommu_dev==NULL special case")
Link: https://lore.kernel.org/all/ab75a444-22a1-47f5-b3c0-253660395b5a@arm.com/
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/20250423164826.2931382-1-arnd@kernel.org
Signed-off-by: Joerg Roedel <jroedel@suse.de>

Author: arndb

drivers/iommu/io-pgtable-arm.c

@@ -13,6 +13,7 @@
 #include <linux/bitops.h>
 #include <linux/io-pgtable.h>
 #include <linux/kernel.h>
+#include <linux/device/faux.h>
 #include <linux/sizes.h>
 #include <linux/slab.h>
 #include <linux/types.h>
@@ -1437,15 +1438,17 @@ static int __init arm_lpae_do_selftests(void)
 	};
 
 	int i, j, k, pass = 0, fail = 0;
-	struct device dev;
+	struct faux_device *dev;
 	struct io_pgtable_cfg cfg = {
 		.tlb = &dummy_tlb_ops,
 		.coherent_walk = true,
-		.iommu_dev = &dev,
 	};
 
-	/* __arm_lpae_alloc_pages() merely needs dev_to_node() to work */
-	set_dev_node(&dev, NUMA_NO_NODE);
+	dev = faux_device_create("io-pgtable-test", NULL, 0);
+	if (!dev)
+		return -ENOMEM;
+
+	cfg.iommu_dev = &dev->dev;
 
 	for (i = 0; i < ARRAY_SIZE(pgsize); ++i) {
 		for (j = 0; j < ARRAY_SIZE(address_size); ++j) {
@@ -1465,6 +1468,8 @@ static int __init arm_lpae_do_selftests(void)
 	}
 
 	pr_info("selftest: completed with %d PASS %d FAIL\n", pass, fail);
+	faux_device_destroy(dev);
+
 	return fail ? -EFAULT : 0;
 }
 subsys_initcall(arm_lpae_do_selftests);