io_uring: port to struct kmem_cache_args

Port req_cachep to struct kmem_cache_args.

Reviewed-by: Kees Cook <kees@kernel.org>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>