USB: usbtmc: prevent kernel-usb-infoleak

The syzbot reported a kernel-usb-infoleak in usbtmc_write,
we need to clear the structure before filling fields.

Fixes: 4ddc645f40e9 ("usb: usbtmc: Add ioctl for vendor specific write")
Reported-and-tested-by: [e-mail]
Closes: https://syzkaller.appspot.com/bug?extid=9d34f80f841e948c3fdb
Signed-off-by: Edward Adam Davis <[e-mail]>
Cc: stable <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/class/usbtmc.c

···
 	if (!urb)
 		return NULL;
 
-	dmabuf = kmalloc(bufsize, GFP_KERNEL);
+	dmabuf = kzalloc(bufsize, GFP_KERNEL);
 	if (!dmabuf) {
 		usb_free_urb(urb);
 		return NULL;