Please cherrypick 592acbf1682128 to 3.18.y and 4.4.y

[Mark Salyzyn]

Cherry pick security-related fix 
592acbf16821288ecdc4192c47e3774a4c48bb64 ("ext4: zero out the unused 
memory region in the extent tree block") to 3.18.y and 4.4.y

The cherry-pick is clean and requires no back-porting. Is already 
present in 4.9.y+

Signed-off-by: Mark Salyzyn<salyzyn@xxxxxxxxxxx>

To: stable <stable@xxxxxxxxxxxxxxx> # 3.18.y 4.4.y

Cc: LKML <linux-kernel@xxxxxxxxxxxxxxx>