ext4: add a structure which will be used by 64bit-resize interface

This patch adds a structure which will be used by 64bit-resize interface.
Two functions which allocate and destroy the structure respectively are
added.

Signed-off-by: Yongqiang Yang <xiaoqiangnk@gmail.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>