rnbd: no need to set bi_end_io in rnbd_bio_map_kern

Since we always set bi_end_io after call rnbd_bio_map_kern, so the
setting in rnbd_bio_map_kern is redundant.

Signed-off-by: Guoqing Jiang <guoqing.jiang@cloud.ionos.com>
Acked-by: Danil Kipnis <danil.kipnis@cloud.ionos.com>
Acked-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>