devmapper: use RAMInBytes() rather than FromHumanSize()

Device Mapper needs device sizes in binary (1024) multiples.  Otherwise
kernel checks can find that the specified thin-pool device sizes aren't
a multiple of the specified thin-pool blocksize.

The name for "RAMInBytes" is likely too narrow given the new consumers
but... Also add "tebibyte" support to RAMInBytes.

Docker-DCO-1.1-Signed-off-by: Mike Snitzer <snitzer@redhat.com> (github: snitm)
devmapper: Add option for specifying the thin pool blocksize

Add dm.blocksize option that you can use with --storage-opt to set a
specific blocksize for the thin provisioning pool.

Also change the default dm-thin-pool blocksize from 64K to 512K.  This
strikes a balance between the desire to have smaller blocksize given
docker's use of snapshots versus the desire to have more performance
that comes with using a larger blocksize.  But if very small files will
be used on average the user is encouraged to override this default.

Docker-DCO-1.1-Signed-off-by: Mike Snitzer <snitzer@redhat.com> (github: snitm)