devmapper: Increase sleep times and unlock while sleeping

We've seen some cases in the wild where waiting for unmount/deactivate
of devmapper devices taking a long time (several seconds). So, we increase
the sleeps to 10 seconds before we timeout. For instance:
 
https://github.com/dotcloud/docker/issues/4389
 
But, in order to not keep other processes blocked we unlock the global
dm lock while waiting to allow other devices to continue working.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)