io_uring: support using a registered personality for commands

For personalities previously registered via IORING_REGISTER_PERSONALITY,
allow any command to select them. This is done through setting
sqe->personality to the id returned from registration, and then flagging
sqe->flags with IOSQE_PERSONALITY.

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

include/uapi/linux/io_uring.h

@@ -40,7 +40,12 @@ struct io_uring_sqe {
 	};
 	__u64	user_data;	/* data to be passed back at completion time */
 	union {
-		__u16	buf_index;	/* index into fixed buffers, if used */
+		struct {
+			/* index into fixed buffers, if used */
+			__u16	buf_index;
+			/* personality to use, if used */
+			__u16	personality;
+		};
 		__u64	__pad2[3];
 	};
 };