ext4: create a new function search_dir

search_dirblock is used to search a dir block, but the code is almost
the same for searching an inline dir.

So create a new fuction search_dir and let search_dirblock call it.

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