drbd: use drbd_device_post_work() in more places

This replaces the md_sync_work member of struct drbd_device
by a new MD_SYNC "work bit" in device->flags.

This replaces the resync_start_work member of struct drbd_device
by a new RS_START "work bit" in device->flags.

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