ext3: improve subtree implementation

We do some cleanups and make code more readable.

*NOTE*
Currently subtree code has a bug that cause quota accounting inaccuretly.
But unfortunately I can't reproduct it until now.  So just use it first.

Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>
ext4: add subtree support

All subtree code are from ext3 subtree implementation.

*NOTE*
Like ext3, currently ext4 subtree implementation also has a bug that
causes quota accounting inaccurately on some unknown scenarios.

Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>