USB: core: urb: Use struct_size() in kmalloc()

["Gustavo A. R. Silva" <gustavo@embeddedor.com>]

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;
    void *entry[];
};

instance = kmalloc(sizeof(struct foo) + sizeof(void *) * count, GFP_KERNEL);

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

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

This code was detected with the help of Coccinelle.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
---
 drivers/usb/core/urb.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/drivers/usb/core/urb.c b/drivers/usb/core/urb.c
index f51750bcd152..0eab79f82ce4 100644
--- a/drivers/usb/core/urb.c
+++ b/drivers/usb/core/urb.c
@@ -70,9 +70,8 @@ struct urb *usb_alloc_urb(int iso_packets, gfp_t mem_flags)
 {
 	struct urb *urb;
 
-	urb = kmalloc(sizeof(struct urb) +
-		iso_packets * sizeof(struct usb_iso_packet_descriptor),
-		mem_flags);
+	urb = kmalloc(struct_size(urb, iso_frame_desc, iso_packets),
+		      mem_flags);
 	if (!urb)
 		return NULL;
 	usb_init_urb(urb);