io_uring/kbuf: use struct io_br_sel for multiple buffers picking

The networking side uses bundles, which is picking multiple buffers at
the same time. Pass in struct io_br_sel to those helpers.

Link: https://lore.kernel.org/r/20250821020750.598432-9-axboe@kernel.dk


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