drbd: Fix a potential write ordering issue on SyncTarget nodes

If a SyncTarget node gets a P_RS_DATA_REPLY before a P_DATA packet
for the same sector, it simply submits these two IO requests.

  This is be possible because on the SyncSource node, the data of the
  P_RS_DATA_REPLY packet was read from disk.  Immediately after that a
  write request from upper layers came in.

The disk scheduler or even the "hardware" queues on the disk drive might
reorder these writes.

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