Previously the rnbd client requested the rtrs to allocate rnbd_iu
just after the rtrs_iu. So the rnbd client passes the size of
rnbd_iu for rtrs_clt_open() and rtrs creates an array of
rnbd_iu and rtrs_iu.

For IO handling, rnbd_iu exists after the request because we pass
the size of rnbd_iu when setting the tag-set. Therefore we do not
use the rnbd_iu allocated by rtrs for IO handling.
We only use the rnbd_iu allocated by rtrs when doing session
initialization. Almost all rnbd_iu allocated by rtrs are wasted.

By this patch the rnbd client does not request rnbd_iu allocation
to rtrs but allocate it for itself when doing session initialization.

Also remove unused rtrs_permit_to_pdu from rtrs.
Signed-off-by: Gioh Kim <gi-oh.kim@cloud.ionos.com>
Signed-off-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>