ext4: fail ext4_iget for root directory if unallocated

ext4: fail ext4_iget for root directory if unallocated

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>
Cc: stable@vger.kernel.org
(cherry picked from commit 8e4b5eae5decd9dfe5a4ee369c22028f90ab4c44)

Orabug: 29048557
CVE: CVE-2018-1092

Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Signed-off-by: Allen Pais <allen.pais@oracle.com>
The current patch borrows EFSBADCRC & EFSCORRUPTED flags from the patch below
6a797d27: ext4: call out CRC and corruption errors with specific error codes

Signed-off-by: Brian Maly <brian.maly@oracle.com>