md: export md_is_rdwr() and is_md_suspended()

The two apis will be used later to fix a deadlock in raid456, there are
no functional changes.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20230512015610.821290-4-yukuai1@huaweicloud.com