md: ensure child flush IO does not affect origin bio->bi_status

When a flush is issued to an RAID array, a child flush IO is created and
issued for each member disk in the RAID array. Since commit b75197e86e6d
("md: Remove flush handling"), each child flush IO has been chained with
the original bio. As a result, the failure of any child IO could modify
the bi_status of the original bio, potentially impacting the upper-layer
filesystem.

Fix the issue by preventing child flush IO from altering the original
bio->bi_status as before. However, this design introduces a known
issue: in the event of a power failure, if a flush IO on a member
disk fails, the upper layers may not be informed. This issue is not easy
to fix and will not be addressed for the time being in this issue.

Fixes: b75197e86e6d ("md: Remove flush handling")
Signed-off-by: Li Nan <[email protected]>
Reviewed-by: Yu Kuai <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Song Liu <[email protected]>

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