Relax restrictions on when --add is permitted.

Relax restrictions on when --add is permitted.

The restriction that --add was not allowed on a device which
looked like a recent member of an array was overly harsh.

The real requirement was to avoid using --add when the array had
failed, and the device being added might contain necessary
information which can only be incorporated by stopping and
re-assembling with --force.

So change the test to reflect the need.

Reported-by: Doug Ledford <dledford@redhat.com>
Signed-off-by: NeilBrown <neilb@suse.de>