ext4: fail ext4_iget for root directory if unallocated

ext4: fail ext4_iget for root directory if unallocated

commit 8e4b5eae5decd9dfe5a4ee369c22028f90ab4c44 upstream.

If the root directory has an i_links_count of zero, then when the file
system is mounted, then when ext4_fill_super() notices the problem and
tries to call iput() the root directory in the error return path,
ext4_evict_inode() will try to free the inode on disk, before all of
the file system structures are set up, and this will result in an OOPS
caused by a NULL pointer dereference.

This issue has been assigned CVE-2018-1092.

https://bugzilla.kernel.org/show_bug.cgi?id=199179
https://bugzilla.redhat.com/show_bug.cgi?id=1560777

Reported-by: Wen Xu <wen.xu@gatech.edu>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
[bwh: Backported to 3.2:
 - Use EIO instead of EFSCORRUPTED
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>