cdev_init: zero out cdev before kobject_init()

Right now, cdev_init() works in a way that is not very intuitive. If a driver passes an uninitialized struct cdev to cdev_init(), then an uninitialized struct kobject will be passed to kobject_init(), which does kset_get() on kobj->kset, which probably points off into space and causes an oops. Drivers can work around this by zeroing out their struct cdev in advance (and indeed most if not all of the things passed to cdev_init() come from BSS) but I think it makes more sense for cdev_init() to live up to its name and actually work on an uninitialized cdev.

Index: x/fs/char_dev.c
===================================================================
@@ -417,6 +417,7 @@
 
 void cdev_init(struct cdev *cdev, struct file_operations *fops)
 {
+	memset(cdev, 0, sizeof *cdev);	
 	INIT_LIST_HEAD(&cdev->list);
 	cdev->kobj.ktype = &ktype_cdev_default;
 	kobject_init(&cdev->kobj);