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

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

commit bc890a60247171294acc0bd67d211fa4b88d40ba 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>
Cc: stable@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit d69a9df614fc68741efcb0fcc020f05caa99d668)

Orabug: 29396712
CVE:CVE-2018-10877

Signed-off-by: John Donnelly <John.P.Donnelly@oracle.com>
Reviewed-by: Somasundaram Krishnasamy <somasundaram.krishnasamy@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>