Fix dm-mod autoloading logic to not assume control node is set correctly.

Fix dm-mod autoloading logic to not assume control node is set correctly.

We can't rely on the fact that udev should prepare the node with right major
and minor number to trigger the module autoloading. We have to take into
account that the node could be missing or it could exist with improper
major and minor number assigned (e.g. from previous kernel versions in
an environment with static nodes and without udev). Make any corrections
if needed!