ext4: factor out is_special_ino()

Factor out the helper is_special_ino() to facilitate the checking of
special inodes in the subsequent patches.

Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Link: https://patch.msgid.link/20250423085257.122685-7-yi.zhang@huaweicloud.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>