Grow/raid10: support reducing the devices in a RAID10.

Grow/raid10: support reducing the devices in a RAID10.

When reducing the number of devices in a RAID10, we increase the
data offset to avoid the need for backup area.

If there is no room at the end of the device to allow this, we need
to first reduce the component size of each device.  However if there
is room, we don't want to insist on that, otherwise growing then
shrinking the array would not be idempotent.

So find the min before/after space before analysing a RAID10 for
reshape, and if the after space is insufficient, reduce the total size
of the array and the component size accordingly.

Signed-off-by: NeilBrown <neilb@suse.de>