devmapper: Enable skip_block_zeroing

This makes the device mapper not zero out blocks allocated on the
thinp device. This is safe in our use case, as we access the device
via a filesystem that doesn't leak any uninitialized data to userspace.

This partially helps with https://github.com/dotcloud/docker/issues/3280
and should generally improve preformance on the devicemapper backend.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
Merge pull request #4017 from alexlarsson/skip_block_zeroing

devmapper: Enable skip_block_zeroing