devmapper: Don't enable discard during mkfs.ext4

There is no reason to do discard durink mkfs, as the filesystem
is on a newly allocated device anyway. Discard is a slow operation,
so this may help initial startup a bit, especially if you use a larger
thin pool.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)