io_uring/memmap: add IO_REGION_F_SINGLE_REF

Kernel allocated compound pages will have just one reference for the
entire page array, add a flag telling io_free_region about that.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/a7abfa7535e9728d5fcade29a1ea1605ec2c04ce.1732886067.git.asml.silence@gmail.com


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