drbd: Remove unnecessary reference counting left-over

Nothing in this function accesses mdev->tconn->net_conf, so there is no
need for get_net_conf() / put_net_conf() anymore.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>