Detail: fix handling of 'disks' array.

Detail: fix handling of 'disks' array.

Since the introduction of replacement devices, we reserve
to places in the "disks" array for each raid disk.
That means we should allocate to twice "max_disk" as the array
could have that many raid_disks (though that would limit the
number of replacements).

A couple of other places need to use "max_disks*2" instead of
"max_disks" to co-ordinate with this.

Reported-by: Or Sagi <ors@reduxio.com>
Signed-off-by: NeilBrown <neilb@suse.de>