With the existing check, if a multipath device gets created with a
blacklisted path (because, for instance, the path was unblacklisted,
but multipathd was not reconfigured), multipathd will crash. This is
because multipathd will add the path when it adds the multipath device,
but it won't have all the necessary information to use the path. The
new check makes sure multipathd won't add blacklisted paths, simply
because they are part of a multipath device.