ext4: let ext4_group_add() use common code

This patch lets ext4_group_add() call ext4_flex_group_add().

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

Conflicts:
	fs/ext4/resize.c