growroot support for dm-crypt encrypted root partitions

I am running RHEL HVM instances in Amazon EC2. We have had to use this package in order to have our root volumes over the default 6GB because of a bug. It has been working really well until we started trying to do this with our encrypted root volumes.

I added some debugging to the dracut module, here is what I see....

I have not dug too much into the problem, is this a known limitation of the package?

I don't mind working to fix it, just didn't want to spin my wheels.

```
_growroot

+ _growroot

dracut: Mounted root filesystem /dev/mapper/cryptroot
+ readlink -f /dev/mapper/cryptroot

+ rootdev=/dev/dm-0

+ [ -z /dev/dm-0 ]

+ [ -e /sys/block/dm-0 ]

+ _info Kevin Debug: /dev/dm-0 is not a partition

+ echo growroot: /dev/dm-0 is not a partition

growroot: /dev/dm-0 is not a partition

+ return
```