mtd: sa1100: avoid VLA in sa1100_setup_mtd

Enabling -Wvla found another variable-length array with randconfig
testing:

drivers/mtd/maps/sa1100-flash.c: In function 'sa1100_setup_mtd':
drivers/mtd/maps/sa1100-flash.c:224:10: error: ISO C90 forbids variable length array 'cdev' [-Werror=vla]

Dynamically allocate the cdev array passed to mtd_concat_create()
instead of using a VLA.
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Olof Johansson <olof@lixom.net>

drivers/mtd/maps/sa1100-flash.c

@@ -221,7 +221,14 @@ static struct sa_info *sa1100_setup_mtd(struct platform_device *pdev,
 		info->mtd = info->subdev[0].mtd;
 		ret = 0;
 	} else if (info->num_subdev > 1) {
-		struct mtd_info *cdev[nr];
+		struct mtd_info **cdev;
+
+		cdev = kmalloc_array(nr, sizeof(*cdev), GFP_KERNEL);
+		if (!cdev) {
+			ret = -ENOMEM;
+			goto err;
+		}
+
 		/*
 		 * We detected multiple devices.  Concatenate them together.
 		 */
@@ -230,6 +237,7 @@ static struct sa_info *sa1100_setup_mtd(struct platform_device *pdev,
 
 		info->mtd = mtd_concat_create(cdev, info->num_subdev,
 					      plat->name);
+		kfree(cdev);
 		if (info->mtd == NULL) {
 			ret = -ENXIO;
 			goto err;