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>