drbd: fix connect failure with all default net-options

If no net-options are configured (all on their default),
no DRBD_NLA_NET_CONF will be passed to the kernel.
The kernel must not require its presence,
there is no required option in there.

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