ext4: change ext4_read_inline_dir() to return 0 on success

In ext4_read_inline_dir(), if there is inline data, the successful
return value is the return value of ext4_read_inline_data().  Howewer,
this is used by ext4_readdir(), and while it seems harmless to return
a positive value on success, it's inconsistent, since historically
we've always return 0 on success.

Signed-off-by: BoxiLiu <lewis.liulei@huawei.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Acked-by: Tao Ma <boyu.mt@taobao.com>