io_uring: introduce struct io_ctx_config

io_uring: introduce struct io_ctx_config

There will be more information needed during ctx setup, and instead of
passing a handful of pointers around, wrap them all into a new
structure. Add a helper for encapsulating all configuration checks and
preparation, that's also reused for ring resizing.

Note, it indirectly adds a io_uring_sanitise_params() check to ring
resizing, which is a good thing.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>