ext4: add a function which extends a group without checking parameters

This patch added a function named ext4_group_extend_no_check() whose code
is copied from ext4_group_extend().  ext4_group_extend_no_check() assumes
the parameter is valid and has been checked by caller.

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