ext4: let ext4_group_extend() use common code

ext4_group_extend_no_check() is moved out from ext4_group_extend(),
this patch lets ext4_group_extend() call ext4_group_extentd_no_check()
instead.

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