drbd: consolidate explicit drbd_md_sync into drbd_create_new_uuid

Every code path changing the current UUID needs to get it on stable
storage anyways. Flush it to disk right there, remove the now obsolte
explicit drbd_md_sync statements in the other code paths.

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