drbd: fix conn_reconfig_start without conn_reconfig_done in drbd_adm_attach

If drbd_adm_attach failed early, it left the CONFIG_PENDING bit on,
blocking any further conn_reconfig_start on that connection.

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