Android Encryption porting
I assume you use the wrong parameter in your fstab. You can't place the footer in space already reserved for a filesystem. Either make an extra partition (sdc5) and specify footer=/dev/sdc5:0 (to put it at the beginning of this partition) or specify a negative offset to use the space at the end of the partition, e.g. to use 16k of the partition specify: encryptable=footer,length=-16384

Not sure if you'd need to make your ext4 filesystem not smaller than the partition, otherwise it could overwrite your footer.