md/raid10: fix memleak for 'conf->bio_split'

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6L4UU


CVE: NA

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In the error path of raid10_run(), 'conf' need be freed, however,
'conf->bio_split' is missed and memory will be leaked.

Since there are 3 places to free 'conf', factor out a helper to fix the
problem.

Fixes: fc9977dd ("md/raid10: simplify the splitting of requests.")
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Jialin Zhang <zhangjialin11@huawei.com>