mdadm: fix --grow with --add for linear

mdadm: fix --grow with --add for linear

For the case mdadm --grow with --add, the s.btype should not be
initialized yet, hence BitmapUnknown should be checked instead of
BitmapNone.

Noted that this behaviour should only support by md-linear, which is
removed from kernel, howerver, it turns out md-linear is used widely
in home NAS and we're planning to reintroduce it soon.

Fixes: 581ba1341017 ("mdadm: remove bitmap file support")
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Mariusz Tkaczyk <mtkaczyk@kernel.org>