pluging in a missing raid member does not (re)add it to array

[ceg]

12.04.1 regression (worked before, maybe without the mentioned safety check, but it worked)

Depends on: Bug #1166086

(re-add refers to speeding up the sync using a bitmap)

The --incremental (udev) call refuses to (re)add a temporarily diconnected member back to an already restarted (active) raid. (Even thought the event count clearly shows its state is equal or older than at the time the array was started (run) degraded.)

This seems to have come by an attempt to "fix" bug #557429, without considering the discussion bejond comment 68
https://bugs.launchpad.net/mdadm/+bug/557429/comments/68

And when attempting to do it manually:

# mdadm /dev/md2 --re-add /dev/sda6
mdadm: --re-add for /dev/sda6 to /dev/md2 is not possible
(missing info: "bacause the array has no write intent bitmap")

# mdadm /dev/md2 --add /dev/sda6
mdadm: /dev/sda6 reports being an active member for /dev/md2, but a --re-add fails.
mdadm: not performing --add as that would convert /dev/sda6 in to a spare.
mdadm: To make this a spare, use "mdadm --zero-superblock /dev/sda6" first.

This is not how things are supposed to work in times of hotpluging.

A warning/question if the device that is to be added contains newer data than at its time of failure is appropriate and very important Bug #1166086, but otherwise, get the job of adding that device back to its raid done!

To avoid regression:
Let the "mdadm --incremental" command (re)add members to running arrays automatically again. Not doing so does not guard against running from alternating (diverging) parts of an array anyway (if the devices come up in random order, the both parts get started degraded, when they come up first Bug #1166086), but at least it prevents overwriting on version with the other.