io_uring/kbuf: introduce struct io_br_sel

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.

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