char_dev: pin parent kobject

char_dev: pin parent kobject

In certain cases (for example when a cdev structure is embedded into
another object whose lifetime is controlled by a separate kobject) it is
beneficial to tie lifetime of another object to the lifetime of
character device so that related object is not freed until after
char_dev object is freed.

To achieve this let's pin kobject's parent when doing cdev_add() and
unpin when last reference to cdev structure is being released.

Signed-off-by: Dmitry Torokhov <[email protected]>
Acked-by: Al Viro <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>