md/raid10: optimize fix_read_error

We dereference r10_bio->read_slot too many times in fix_read_error().
Optimize it by using a variable to store read_slot.

Signed-off-by: Li Nan <linan122@huawei.com>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20230623173236.2513554-2-linan666@huaweicloud.com


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