io_uring: allow registering credentials

If an application wants to use a ring with different kinds of
credentials, it can register them upfront. We don't lookup credentials,
the credentials of the task calling IORING_REGISTER_PERSONALITY is used.

An 'id' is returned for the application to use in subsequent personality
support.

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

include/uapi/linux/io_uring.h

@@ -211,6 +211,8 @@ struct io_uring_params {
 #define IORING_REGISTER_FILES_UPDATE	6
 #define IORING_REGISTER_EVENTFD_ASYNC	7
 #define IORING_REGISTER_PROBE		8
+#define IORING_REGISTER_PERSONALITY	9
+#define IORING_UNREGISTER_PERSONALITY	10
 
 struct io_uring_files_update {
 	__u32 offset;