md: replace is_md_suspended() with 'mddev->suspended' in md_check_recovery()

Prepare to cleanup pers->prepare_suspend(), which is used to fix a
deadlock in raid456 by returning error for io that is waiting for
reshape to make progress in mddev_suspend().

This change will allow reshape to make progress while waiting for io to
be done in mddev_suspend() in following patches.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20231010151958.145896-4-yukuai1@huaweicloud.com