When hiddev_disconnect() runs with chardev open, it will proceed with
usbhid_close(). When userspace in parallel runs the hiddev_release(),
it sees !hiddev->exists (as it has been already set so by
hiddev_disconnect()) and kfrees hiddev while hiddev_disconnect() hasn't
finished yet.

Serialize the access to hiddev->exists and hiddev->open by existancelock.

Reported-by: mike-@cinci.rr.com
Signed-off-by: Jiri Kosina <jkosina@suse.cz>