ext4: add a new function which adds a flex group to a fs

This patch adds a new function named ext4_flex_group_add() which adds a
flex group to a fs.  The function is used by 64bit-resize interface.

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