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

[Greg Hackmann <ghackmann@xxxxxxxxxx>]

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.