Don't let incremental add devices to active arrays.

Don't let incremental add devices to active arrays.

Adding devices to active arrays in --incremental is a bit dubious.

Normally the array won't be activated until all expected devices are
present, so this situation would mean that the given device is not
expected, so is probably failed.  In that case it should only be added
by explicit sysadmin request.

However if --run was given, then quite possibly the array was
assembled earlier when not complete, so it is less clear whether it is
wrong to add this device or not.  In that case add it as that is
generally safest.

It would be nice to allow policy for this to be explicitly given by
sysadmin.

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