rnbd-srv: Fix server side setting of bi_size for special IOs

rnbd-srv: Fix server side setting of bi_size for special IOs

On rnbd-srv, the bi_size of the bio is set during the bio_add_page
function, to which datalen is passed. But for special IOs like DISCARD
and WRITE_ZEROES, datalen is 0, since there is no data to write. For
these special IOs, use the bi_size of the rnbd_msg_io.

Fixes: f6f84be089c9 ("block/rnbd-srv: Add sanity check and remove redundant assignment")
Signed-off-by: Florian-Ewald Mueller <florian-ewald.mueller@ionos.com>
Signed-off-by: Md Haris Iqbal <haris.iqbal@ionos.com>
Signed-off-by: Grzegorz Prajsner <grzegorz.prajsner@ionos.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>