md/raid1: use rdev in raid1_write_request directly

We already get rdev from conf->mirrors[i].rdev at the beginning of the
loop, so just use it.

Signed-off-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>