nbd: add a flush_workqueue in nbd_start_device

mainline inclusion
from mainline-v5.6-rc1
commit 5c0dd228
category: bugfix
bugzilla: 185690
CVE: NA

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When kzalloc fail, may cause trying to destroy the
workqueue from inside the workqueue.

If num_connections is m (2 < m), and NO.1 ~ NO.n
(1 < n < m) kzalloc are successful. The NO.(n + 1)
failed. Then, nbd_start_device will return ENOMEM
to nbd_start_device_ioctl, and nbd_start_device_ioctl
will return immediately without running flush_workqueue.
However, we still have n recv threads. If nbd_release
run first, recv threads may have to drop the last
config_refs and try to destroy the workqueue from
inside the workqueue.

To fix it, add a flush_workqueue in nbd_start_device.

Fixes: e9e006f5 ("nbd: fix max number of supported devs")
Signed-off-by: Sun Ke <sunke32@huawei.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Ye Bin <yebin10@huawei.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>