CVE-2024-42304 ext4: make sure the first directory block is not a hole

In the Linux kernel, the following vulnerability has been resolved:

ext4: make sure the first directory block is not a hole

The syzbot constructs a directory that has no dirblock but is non-inline,
i.e. the first directory block is a hole. And no errors are reported when
creating files in this directory in the following flow.

ext4_mknod
 ...
  ext4_add_entry
    // Read block 0
    ext4_read_dirblock(dir, block, DIRENT)
      bh = ext4_bread(NULL, inode, block, 0)
      if (!bh && (type == INDEX || type == DIRENT_HTREE))
      // The first directory block is a hole
      // But type == DIRENT, so no error is reported.

After that, we get a directory block without ‘.’ and ‘…’ but with a valid
dentry. This may cause some code that relies on dot or dotdot (such as
make_indexed_dir()) to crash.

Therefore when ext4_read_dirblock() finds that the first directory block
is a hole report that the filesystem is corrupted and return an error to
avoid loading corrupted data from disk causing something bad.