md: do not return existing mddevs from mddev_find_or_alloc

mainline inclusion
from mainline-v5.13-rc1
commit 0d809b38
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I8O6NL
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=0d809b3837a0bede8f58a67e303e339585777bf4



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Instead of returning an existing mddev, just for it to be discarded
later directly return -EEXIST.  Rename the function to mddev_alloc now
that it doesn't find an existing mddev.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Song Liu <song@kernel.org>

Conflicts:
  Commit 4ad5f208 ("md: Flush workqueue md_rdev_misc_wq in md_alloc()")
  add flushing md_rdev_misc_wq in md_alloc().
Signed-off-by: Li Lingfeng <lilingfeng3@huawei.com>