Please apply "ext4: verify the depth of extent tree in ext4_find_extent()" to 3.18.y

bc890a602471 ("ext4: verify the depth of extent tree in ext4_find_extent()") fixes a potential buffer overrun when mounting corrupted ext4 filesystems.

It was taken into 4.4.y and later but not 3.18.y, probably because it doesn't build as-is against 3.18. Ben Hutchings has a backport in the latest 3.16.y release (09999807edd8) which would work for 3.18.y too.