drbd: Fixes to the new delay_probes code

* Only send delay_probes with protocol 93 or newer
* drbd_send_delay_probes() is called only from worker context,
  no atomic_t needed for delay_seq

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