ext4: make ext4_delete_entry generic

Currently ext4_delete_entry() is used only for dir entry removing from
a dir block.  So let us create a new function
ext4_generic_delete_entry and this function takes a entry_buf and a
buf_size so that it can be used for inline data.

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