[PATCH] md: fix --re-add for raid1 and raid6

If you have an array with a write-intent-bitmap, and you remove a device, then
re-add it, a full recovery isn't needed.  We detect a re-add by looking at
saved_raid_disk.  For raid1, it doesn't matter which disk it was, only whether
or not it was an active device.  The old code being removed set a value of
'mirror' which was then ignored, so it can go.  The changed code performs the
correct check.

For raid6, if there are two missing devices, make sure we chose the right slot
on --re-add rather than always the first slot.

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>