drbd: fix NULL pointer deref in module init error path

If we want to iterate over the (as of yet still empty) list in the
cleanup path, we need to initialize the list before the first goto fail.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>