mdadm: refactor write journal code in Assemble and Incremental

mdadm: refactor write journal code in Assemble and Incremental

As discussed, standalone require_journal() in struct superswitch
is not a very good idea. Instead, journal related information
fits well in struct mdinfo.

This patch simplifies journal support code in Assemble and
Incremental as:

- Add journal_device_required and journal_clean to struct mdinfo;
- Remove function require_journal from struct superswitch;
- Update Assemble and Incremental to use journal_device_required
and journal_clean from struct mdinfo (instead of separate var).

Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Shaohua Li <shli@fb.com>
Signed-off-by: NeilBrown <neilb@suse.com>