md/raid10: fix null-ptr-deref of mreplace in raid10_sync_request

hulk inclusion
category: bugfix
bugzilla: 188527, https://gitee.com/openeuler/kernel/issues/I6O3HO


CVE: NA

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need_replace will be set to 1 if no-Faulty mreplace exists, and mreplace
will be deref later. However, the latter check of mreplace might set
mreplace to NULL, null-ptr-deref occurs if need_replace is 1 at this time.

Fix it by merging two checks into one.

Fixes: ee37d731 ("md/raid10: Fix raid10 replace hang when new added disk faulty")
Signed-off-by: Li Nan <linan122@huawei.com>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Hou Tao <houtao1@huawei.com>