Grow: fix problem with --grow --continue

Grow: fix problem with --grow --continue

If an array is being reshaped using backup space on a 'spare' device,
then
  mdadm --grow --continue
won't find it as by the time it runs, nothing looks like a spare are
more.  The spare has been added to the array, but has no data yet.

So allow reshape_prepare_fdlist to find a newly-incorporated spare and
report this so it can be used.

Reported-by: Xiao Ni <xni@redhat.com>
Signed-off-by: NeilBrown <neilb@suse.de>