md/raid10: remove rcu protection to access rdev from conf

mainline inclusion
from mainline-next-20231220
commit a448af25becf4b555660b5ba2618c7ed3c4de6da
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I8T02O

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=a448af25becf4b555660b5ba2618c7ed3c4de6da



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Because it's safe to accees rdev from conf:
 - If any spinlock is held, because synchronize_rcu() from
   md_kick_rdev_from_array() will prevent 'rdev' to be freed until
   spinlock is released;
 - If 'reconfig_lock' is held, because rdev can't be added or removed from
   array;
 - If there is normal IO inflight, because mddev_suspend() will prevent
   rdev to be added or removed from array;
 - If there is sync IO inflight, because 'MD_RECOVERY_RUNNING' is
   checked in remove_and_add_spares().

And these will cover all the scenarios in raid10.

This patch also cleanup the code to handle the case that replacement
replace rdev while IO is still inflight.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20231125081604.3939938-3-yukuai1@huaweicloud.com


Signed-off-by: Li Nan <linan122@huawei.com>