drbd: Turn conn_flush_workqueue() into drbd_flush_workqueue()

The new function can flush any work queue, not just the work queue of the data
socket of a connection.

Signed-off-by: Andreas Gruenbacher <agruen@linbit.com>
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>

diff --cc drivers/block/drbd/drbd_int.h
Simple merge

diff --cc drivers/block/drbd/drbd_main.c
Simple merge

diff --cc drivers/block/drbd/drbd_nl.c
Simple merge

diff --cc drivers/block/drbd/drbd_receiver.c
Simple merge