drbd: Use ping-timeout when waiting for missing ack packets

When the node with the discard flag resolves write conflicts in
dual-primary mode, it may determine that its peer has sent ack packets
on the metadata socket which did not arrive, yet.  Wait for the next ack
with ping-timeout instead of a hard-coded 30 seconds.

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