ext4: add more inode number paranoia checks

If there is a directory entry pointing to a system inode (such as a
journal inode), complain and declare the file system to be corrupted.

Also, if the superblock's first inode number field is too small,
refuse to mount the file system.

This addresses CVE-2018-10882.

https://bugzilla.kernel.org/show_bug.cgi?id=200069



Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org
Git-commit: c37e9e013469521d9adb932d17a1795c139b36db
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Change-Id: Idf9ddc1f491b600136aef41dd240495036c489ee
Signed-off-by: Srinivasa Rao Kuppala <srkupp@codeaurora.org>