Dmthinp devel

This is my first pull request for docker.  I have my flame retardant pants on ;)

I am an upstream kernel maintainer for the Device Mapper (DM) subsystem.
I've been tasked with helping understand docker on dm-thinp performance using enterprise workloads.  The first step in being able to do that is to allow control of the blocksize that is used for docker's underlying dm-thin-pool.

While adding the new 'dm.blocksize' storage option I fixed a few issues along the way.
Can you include a statement on the default blocksize, when no flag is provided.