io_uring: add fully sparse buffer registration

Honour IORING_RSRC_REGISTER_SPARSE not only for direct files but fixed
buffers as well. It makes the rsrc API more consistent.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/66f429e4912fe39fb3318217ff33a2853d4544be.1652879898.git.asml.silence@gmail.com


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