rnbd-srv: simplify bio mapping in process_rdma

The memory mapped in process_rdma is contiguous, so there is no need
to loop over bio_add_page.  Remove rnbd_bio_map_kern and just open code
the bio allocation and mapping in the caller.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jack Wang <jinpu.wang@ionons.com>
Tested-by: Jack Wang <jinpu.wang@ionos.com>
Link: https://lore.kernel.org/r/20220124091107.642561-11-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>