md/raid10: fix io hung in md_wait_for_blocked_rdev()

hulk inclusion
category: bugfix
bugzilla: 188569, https://gitee.com/openeuler/kernel/issues/I6XBZQ


CVE: NA

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If badblocks are merged but bb->count exceedded, badblocks_set() will
return 1 and merged badblocks will become un-ack. rdev_set_badblocks()
will not set sb_flags and wakeup mddev->thread, io wait in
md_wait_for_blocked_rdev() will hung because BlockedBadBlocks may not be
cleared.

Fix it by checking badblocks->changed instead of return value. This flag
is set when badblocks changes.

Signed-off-by: Li Nan <linan122@huawei.com>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Hou Tao <houtao1@huawei.com>