drbd: re-add lost conf_mutex protection in drbd_set_role

The conf_update mutex used to be held while clearing the
net_conf->discard_my_data flag inside drbd_set_role.

It was moved into drbd_adm_set_role with
    drbd: allow parallel promote/demote actions
but then replaced at that location by the newly introduced adm_mutex with
    drbd: Fix a potential deadlock in drbdsetup, introduce resource->adm_mutex

And I simply forgot to put it back in at the original location.
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

drivers/block/drbd/drbd_nl.c

@@ -661,11 +661,11 @@ drbd_set_role(struct drbd_device *const device, enum drbd_role new_role, int for
 			put_ldev(device);
 		}
 	} else {
-		/* Called from drbd_adm_set_role only.
-		 * We are still holding the conf_update mutex. */
+		mutex_lock(&device->resource->conf_update);
 		nc = connection->net_conf;
 		if (nc)
 			nc->discard_my_data = 0; /* without copy; single bit op is atomic */
+		mutex_unlock(&device->resource->conf_update);
 
 		set_disk_ro(device->vdisk, false);
 		if (get_ldev(device)) {