[PATCH 2/3] erofs: convert to use kobject_is_added()

[Yangtao Li]

> Meta-comment, we need to come up with a "filesystem kobject type" to get
> rid of lots of the boilerplate filesystem kobject logic as it's
> duplicated in every filesystem in tiny different ways and lots of times
> (like here), it's wrong.

Can we add the following structure?

struct filesystem_kobject {
        struct kobject kobject;
        struct completion unregister;
};

w/ it, we can simplify something:

1. merge init_completion and kobject_init_and_add

2. merge kobject_put and wait_for_completion

3. we can have a common release func for kobj_type release

MBR,
Yangtao