md: fix kabi broken in struct mddev

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6T6VY


CVE: NA

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Struct mddev is just used inside raid, just in case that md_mod is compiled
from new kernel, and raid1/raid10 or other out-of-tree raid are compiled
from old kernel.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>