io-uring: move `struct io_br_sel` into io_uring_types.h

Move `struct io_br_sel` into io_uring_types.h and prepare for supporting
provided buffer on uring_cmd.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250821040210.1152145-2-ming.lei@redhat.com


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