UPSTREAM: char_dev: pin parent kobject

UPSTREAM: 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.

BUG=chrome-os-partner:15849
TEST=suspend_stress_test

Change-Id: I35d1762c94afa8b3eda6329a0bf0cfc7dde98b7b
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Michael Spang <spang@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/37134
Reviewed-by: Vincent Palatin <vpalatin@chromium.org>