usb: core: config: Use struct_size() in kzalloc()

One of the more common cases of allocation size calculations is finding
the size of a structure that has a zero-sized array at the end, along
with memory for some number of elements for that array. For example:

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

size = sizeof(struct foo) + count * sizeof(struct boo);
instance = kzalloc(size, GFP_KERNEL);

Instead of leaving these open-coded and prone to type mistakes, we can
now use the new struct_size() helper:

instance = kzalloc(struct_size(instance, entry, count), GFP_KERNEL);

Notice that, in this case, variable len is not necessary, hence
it is removed.

This code was detected with the help of Coccinelle.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/core/config.c

@@ -552,7 +552,7 @@ static int usb_parse_configuration(struct usb_device *dev, int cfgidx,
 	unsigned char *buffer2;
 	int size2;
 	struct usb_descriptor_header *header;
-	int len, retval;
+	int retval;
 	u8 inums[USB_MAXINTERFACES], nalts[USB_MAXINTERFACES];
 	unsigned iad_num = 0;
 
@@ -707,8 +707,8 @@ static int usb_parse_configuration(struct usb_device *dev, int cfgidx,
 			nalts[i] = j = USB_MAXALTSETTING;
 		}
 
-		len = sizeof(*intfc) + sizeof(struct usb_host_interface) * j;
-		config->intf_cache[i] = intfc = kzalloc(len, GFP_KERNEL);
+		intfc = kzalloc(struct_size(intfc, altsetting, j), GFP_KERNEL);
+		config->intf_cache[i] = intfc;
 		if (!intfc)
 			return -ENOMEM;
 		kref_init(&intfc->ref);