ext4: implement filesystem specific alloc_inode in unit test

We expect inode with ext4_info_info type as following:
mbt_kunit_init
  mbt_mb_init
    ext4_mb_init
      ext4_mb_init_backend
        sbi->s_buddy_cache = new_inode(sb);
        EXT4_I(sbi->s_buddy_cache)->i_disksize = 0;

Implement alloc_inode ionde with ext4_inode_info type to avoid
out-of-bounds write.

Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20240322165518.8147-1-shikemeng@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>