md: Flush workqueue md_rdev_misc_wq in md_alloc()

mainline inclusion
from mainline-v6.0-rc3
commit 5e8daf90
category: bugfix
bugzilla: 188015, https://gitee.com/openeuler/kernel/issues/I6OERX
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit?id=5e8daf906f890560df430d30617c692a794acb73



--------------------------------

A race condition still exists when removing and re-creating md devices
in test cases. However, it is only seen on some setups.

The race condition was tracked down to a reference still being held
to the kobject by the rdev in the md_rdev_misc_wq which will be released
in rdev_delayed_delete().

md_alloc() waits for previous deletions by waiting on the md_misc_wq,
but the md_rdev_misc_wq may still be holding a reference to a recently
removed device.

To fix this, also flush the md_rdev_misc_wq in md_alloc().

Signed-off-by: David Sloan <david.sloan@eideticom.com>
[logang@deltatee.com: rewrote commit message]
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Song Liu <song@kernel.org>

Conflict:
	drivers/md/md.c

Signed-off-by: Li Nan <linan122@huawei.com>
Reviewed-by: Hou Tao <houtao1@huawei.com>
(cherry picked from commit 5fa41917)