drbd: fix race on meta-data update

The race:
	drbd_md_mark_dirty()
	drbd_md_sync()
		if (!test_and_clear_bit(MD_DIRTY, &mdev->flags))
			return;
		drbd_md_sync_page_io(mdev, mdev->ldev, sector, WRITE)
  ==> RACE
		clear_bit(MD_DIRTY, &mdev->flags); <== spurious

Fixed by removing the spurious clear_bit.

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