wifi: at76c50x: prefer struct_size over open coded arithmetic

This is an effort to get rid of all multiplications from allocation
functions in order to prevent integer overflows [1][2].

As the "cmd_buf" variable is a pointer to "struct at76_command" and
this structure ends in a flexible array:

struct at76_command {
[...]
u8 data[];
} __packed;

the preferred way in the kernel is to use the struct_size() helper to
do the arithmetic instead of the calculation "size + count" in the
kmalloc() function.

Also, declare a new variable (total_size) since the return value of the
struct_size() helper is used several times.

At the same time, prepare for the coming implementation by GCC and Clang
of the __counted_by attribute. Flexible array members annotated with
__counted_by can have their accesses bounds-checked at run-time via
CONFIG_UBSAN_BOUNDS (for array indexing) and CONFIG_FORTIFY_SOURCE (for
strcpy/memcpy-family functions). In this case, it is important to note
that the attribute used is "__counted_by_le" since the counter type is
"__le16".

This way, the code is more readable and safer.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#open-coded-arithmetic-in-allocator-arguments [1]
Link: https://github.com/KSPP/linux/issues/160 [2]
Signed-off-by: Erick Archer <[e-mail]>
Signed-off-by: Kalle Valo <[e-mail]>
Link: https://msgid.link/[e-mail]

drivers/net/wireless/atmel/at76c50x-usb.c

···
 				 int buf_size)
 {
 	int ret;
-	struct at76_command *cmd_buf = kmalloc(sizeof(struct at76_command) +
-					       buf_size, GFP_KERNEL);
+	size_t total_size;
+	struct at76_command *cmd_buf;
 
+	total_size = struct_size(cmd_buf, data, buf_size);
+	cmd_buf = kmalloc(total_size, GFP_KERNEL);
 	if (!cmd_buf)
 		return -ENOMEM;
 
···
 	cmd_buf->size = cpu_to_le16(buf_size);
 	memcpy(cmd_buf->data, buf, buf_size);
 
-	at76_dbg_dump(DBG_CMD, cmd_buf, sizeof(struct at76_command) + buf_size,
+	at76_dbg_dump(DBG_CMD, cmd_buf, total_size,
 		      "issuing command %s (0x%02x)",
 		      at76_get_cmd_string(cmd), cmd);
 
 	ret = usb_control_msg(udev, usb_sndctrlpipe(udev, 0), 0x0e,
 			      USB_TYPE_VENDOR | USB_DIR_OUT | USB_RECIP_DEVICE,
-			      0, 0, cmd_buf,
-			      sizeof(struct at76_command) + buf_size,
-			      USB_CTRL_GET_TIMEOUT);
+			      0, 0, cmd_buf, total_size, USB_CTRL_GET_TIMEOUT);
 	kfree(cmd_buf);
 	return ret;
 }

drivers/net/wireless/atmel/at76c50x-usb.h

···
 	u8 cmd;
 	u8 reserved;
 	__le16 size;
-	u8 data[];
+	u8 data[] __counted_by_le(size);
 } __packed;
 
 /* Length of Atmel-specific Rx header before 802.11 frame */