io_uring: move provided and fixed buffers into the same io_kiocb area

These are mutually exclusive - if you use provided buffers, then you
cannot use fixed buffers and vice versa. Move them into the same spot
in the io_kiocb, which is also advantageous for provided buffers as
they get near the submit side hot cacheline.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -958,8 +958,14 @@ struct io_kiocb {
 	struct task_struct		*task;
 
 	struct io_rsrc_node		*rsrc_node;
-	/* store used ubuf, so we can prevent reloading */
-	struct io_mapped_ubuf		*imu;
+
+	union {
+		/* store used ubuf, so we can prevent reloading */
+		struct io_mapped_ubuf	*imu;
+
+		/* stores selected buf, valid IFF REQ_F_BUFFER_SELECTED is set */
+		struct io_buffer	*kbuf;
+	};
 
 	union {
 		/* used by request caches, completion batching and iopoll */
@@ -976,8 +982,6 @@ struct io_kiocb {
 	struct async_poll		*apoll;
 	/* opcode allocated if it needs to store data for async defer */
 	void				*async_data;
-	/* stores selected buf, valid IFF REQ_F_BUFFER_SELECTED is set */
-	struct io_buffer		*kbuf;
 	/* linked requests, IFF REQ_F_HARDLINK or REQ_F_LINK are set */
 	struct io_kiocb			*link;
 	/* custom credentials, valid IFF REQ_F_CREDS is set */