ext4: let ext4_rename handle inline dir

In case we rename a directory, ext4_rename has to read the dir block
and change its dotdot's information.  The old ext4_rename encapsulated
the dir_block read into itself.  So this patch adds a new function
ext4_get_first_dir_block() which gets the dir buffer information so
the ext4_rename can handle it properly.  As it will also change the
parent inode number, we return the parent_de so that ext4_rename() can
handle it more easily.

ext4_find_entry is also changed so that the caller(rename) can tell
whether the found entry is an inlined one or not and journaling the
corresponding buffer head.

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