Fix a issue which causes subtree based dir quota not to work.

Returning a positive value on success from ext3_subtree_xattr_read() in file
cache building stage would cause a bad inode pointer lays in NULL page, which
in turn crashes the kernel.

Fix it by return 0 on success.

Signed-off-by: Huang Le <le.hl@alibaba-inc.com>
Merge pull request #36 from 4tar/dev

Fix a issue which causes subtree based dir quota not to work.

[Note from Zheng Liu]
The lesson from this is that never touch the code which has running on thousands of servers. :(