md/raid5: fix spares errors about rcu usage

As commit ad860670 ("md/raid5: remove rcu protection to access rdev
from conf") explains, rcu protection can be removed, however, there are
three places left, there won't be any real problems.

drivers/md/raid5.c:8071:24: error: incompatible types in comparison expression (different address spaces):
drivers/md/raid5.c:8071:24:    struct md_rdev [noderef] __rcu *
drivers/md/raid5.c:8071:24:    struct md_rdev *
drivers/md/raid5.c:7569:25: error: incompatible types in comparison expression (different address spaces):
drivers/md/raid5.c:7569:25:    struct md_rdev [noderef] __rcu *
drivers/md/raid5.c:7569:25:    struct md_rdev *
drivers/md/raid5.c:7573:25: error: incompatible types in comparison expression (different address spaces):
drivers/md/raid5.c:7573:25:    struct md_rdev [noderef] __rcu *
drivers/md/raid5.c:7573:25:    struct md_rdev *

Fixes: ad860670 ("md/raid5: remove rcu protection to access rdev from conf")
Cc: stable@vger.kernel.org
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20240615085143.1648223-1-yukuai1@huaweicloud.com