md/raid10: use dereference_rdev_and_rrdev() to get devices

Commit 2ae6aaf7 ("md/raid10: fix io loss while replacement replace
rdev") reads replacement first to prevent io loss. However, there are same
issue in wait_blocked_dev() and raid10_handle_discard(), too. Fix it by
using dereference_rdev_and_rrdev() to get devices.

Fixes: d30588b2 ("md/raid10: improve raid10 discard request")
Fixes: f2e7e269 ("md/raid10: pull the code that wait for blocked dev into one function")
Signed-off-by: Li Nan <linan122@huawei.com>
Link: https://lore.kernel.org/r/20230701080529.2684932-4-linan666@huaweicloud.com


Signed-off-by: Song Liu <song@kernel.org>