ext4: verify the depth of extent tree in ext4_find_extent()

If there is a corupted file system where the claimed depth of the
extent tree is -1, this can cause a massive buffer overrun leading to
sadness.

This addresses CVE-2018-10877.

https://bugzilla.kernel.org/show_bug.cgi?id=199417



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


Git-commit: bc890a60247171294acc0bd67d211fa4b88d40ba
Signed-off-by: Srinivasa Rao Kuppala <srkupp@codeaurora.org>
Change-Id: I9f899cc3e34655ae6b80a3d2720a6a91a43b749f