Check write journal in incremental

Check write journal in incremental

If journal device is missing, do not start the array, and shows:

./mdadm -I /dev/sdf
mdadm: journal device is missing, not safe to start yet.

The array will be started when the journal device is attached with -I

./mdadm -I /dev/sdb1
mdadm: /dev/sdb1 attached to /dev/md/0_0, which has been started.

To force start without journal device:

./mdadm -I /dev/sdf --run
mdadm: Trying to run with missing journal device
mdadm: /dev/sdf attached to /dev/md/0_0, which has been started.

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