md/raid10: fix a race between removing rdev and access conf->mirrors[i].rdev

hulk inclusion
category: bugfix
bugzilla: 188533, https://gitee.com/openeuler/kernel/issues/I6O7YB


CVE: NA

--------------------------------

commit ceff49d9 ("md/raid1: fix a race between removing rdev and
access conf->mirrors[i].rdev") fix a null-ptr-deref about raid1. There
is same bug in raid10 and fix it in the same way.

There is no sync_thread running while removing rdev, no need to check
the flag in raid10_sync_request().

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