Make the function available for outside use and fortify it against NULL
kobject.

CC: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: NBart Van Assche <bart.vanassche@sandisk.com>
Acked-by: NTejun Heo <tj@kernel.org>
Signed-off-by: NJan Kara <jack@suse.cz>
Signed-off-by: NJens Axboe <axboe@fb.com>