devmapper: Drop devices lock before returning from function

cleanupDeleted() takes devices.Lock() but does not drop it if there are
no deleted devices. Hence docker deadlocks if one is using deferred
device deletion feature. (--storage-opt dm.use_deferred_deletion=true).

Fix it. Drop the lock before returning.

Also added a unit test case to make sure in future this can be easily
detected if somebody changes the function.

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