io_uring/kbuf: introduce struct io_br_sel

Rather than return addresses directly from buffer selection, add a
struct around it. No functional changes in this patch, it's in
preparation for storing more buffer related information locally, rather
than in struct io_kiocb.

Link: https://lore.kernel.org/r/20250821020750.598432-7-axboe@kernel.dk


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