Incremental: allow recently removed device to be added as a spare.

Incremental: allow recently removed device to be added as a spare.

Currently, action=force-spare isn't effective at all as I'm not
sure what is really sensible.
This patch allows a device that was part of an array, but has been
removed, to be added as a spare of passed to --incremental while
force-spare is active.

If it is can be re-added, that done first.  If it fails, we add it as
a spare.

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