devmapper: Log start and end of filesystem creation

ext4 filesystem creation can take a long time on 100G thin device and
systemd might time out and kill docker service. Often user is left thinking
why docker is taking so long and logs don't give any hint. Log an info
message in journal for start and end of filesystem creation. That way
a user can look at logs and figure out that filesystem creation is
taking long time.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>