io_uring: simplify struct io_uring_sqe layout

Flatten struct io_uring_sqe, the last union is exactly 64B, so move them
out of union { struct { ... }}, and decrease __pad2 size.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/2e21ef7aed136293d654450bc3088973a8adc730.1624543113.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

include/uapi/linux/io_uring.h

@@ -46,8 +46,6 @@ struct io_uring_sqe {
 		__u32		unlink_flags;
 	};
 	__u64	user_data;	/* data to be passed back at completion time */
-	union {
-		struct {
 	/* pack this to avoid bogus arm OABI complaints */
 	union {
 		/* index into fixed buffers, if used */
@@ -58,9 +56,7 @@ struct io_uring_sqe {
 	/* personality to use, if used */
 	__u16	personality;
 	__s32	splice_fd_in;
-		};
-		__u64	__pad2[3];
-	};
+	__u64	__pad2[2];
 };
 
 enum {