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

commit bc890a60 upstream.

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>
[bwh: Backported to 3.16: return -EIO instead of -EFSCORRUPTED]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Greg Hackmann <ghackmann@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>