ACPICA: Replace one-element array with flexible-array

ACPICA commit 7ba2f3d91a32f104765961fda0ed78b884ae193d

The current codebase makes use of one-element arrays in the following
form:

struct something {
    int length;
    u8 data[1];
};

struct something *instance;

instance = kmalloc(sizeof(*instance) + size, GFP_KERNEL);
instance->length = size;
memcpy(instance->data, source, size);

but the preferred mechanism to declare variable-length types such as
these ones is a flexible array member[1][2], introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure,
which will help us prevent some kind of undefined behavior bugs from
being inadvertently introduced[3] to the linux codebase from now on.

This issue was found with the help of Coccinelle and audited _manually_.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732 ("cxgb3/l2t: Fix undefined behaviour")

Link: https://github.com/acpica/acpica/commit/7ba2f3d9Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Erik Kaneda <erik.kaneda@intel.com>
Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/acpica/utids.c

@@ -263,8 +263,7 @@ acpi_ut_execute_CID(struct acpi_namespace_node *device_node,
 	 * 3) Size of the actual CID strings
 	 */
 	cid_list_size = sizeof(struct acpi_pnp_device_id_list) +
-	    ((count - 1) * sizeof(struct acpi_pnp_device_id)) +
-	    string_area_size;
+	    (count * sizeof(struct acpi_pnp_device_id)) + string_area_size;
 
 	cid_list = ACPI_ALLOCATE_ZEROED(cid_list_size);
 	if (!cid_list) {

include/acpi/actypes.h

@@ -1146,7 +1146,7 @@ struct acpi_pnp_device_id {
 struct acpi_pnp_device_id_list {
 	u32 count;		/* Number of IDs in Ids array */
 	u32 list_size;		/* Size of list, including ID strings */
-	struct acpi_pnp_device_id ids[1];	/* ID array */
+	struct acpi_pnp_device_id ids[];	/* ID array */
 };
 
 /*