md: replace invalid function flush_rdev_wq() with flush_workqueue()

hulk inclusion
category: bugfix
bugzilla: 188553, https://gitee.com/openeuler/kernel/issues/I6TNFX


CVE: NA

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If we want to remove a device, first we delete it from mddev->disks list,
then init rdev->del_work to put it (see unbind_rdev_from_array()).

flush_rdev_wq() traverses mddev->disks to check if there is any pending
rdev->del_work, if so, flush it. Howerver, rdev will not be in the list of
mddev->disks if rdev->del_work exists, and flush_workqueue() will never be
executed.

Replace it with flush_workqueue() to ensure del_work has been completed
when adding devices.

Fixes: cc1ffe61 ("md: add new workqueue for delete rdev")
Signed-off-by: Li Nan <linan122@huawei.com>
Reviewed-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>