md: factor out a mddev_alloc_unit helper from mddev_find

mainline inclusion
from mainline-v5.13-rc1
commit 85c8c3c1
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=85c8c3c1f8d9e31f626c93435dd91c2f85603e07



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Split out a self contained helper to find a free minor for the md
"unit" number.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Song Liu <song@kernel.org>
Signed-off-by: Li Lingfeng <lilingfeng3@huawei.com>