md: Constify attribute_group structs

mainline inclusion
from mainline-v5.14-rc1
commit c32dc040
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I81XCK

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c32dc04059c79ddb4f7cff94ad5de6e92ea2218d



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The attribute_group structs are never modified, they're only passed to
sysfs_create_group() and sysfs_remove_group(). Make them const to allow
the compiler to put them in read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Song Liu <song@kernel.org>
Signed-off-by: Li Nan <linan122@huawei.com>