md/raid10: check replacement and rdev to prevent submit the same io twice

After commit 4ca40c2c ("md/raid10: Allow replacement device to be
replace old drive."), 'rdev' and 'replacement' could appear to be
identical. There are already checks for that in wait_blocked_dev() and
raid10_write_request(). Add check for raid10_handle_discard() now.

Signed-off-by: Li Nan <linan122@huawei.com>
Link: https://lore.kernel.org/r/20230701080529.2684932-2-linan666@huaweicloud.com


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