Revert "io_uring: add buffer selection support to IORING_OP_NOP"

This reverts commit 3d20024.

Buffer selection with nops was used for debugging and benchmarking but
is useless in real life. Let's revert it before it's released.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/c5012098ca6b51dfbdcb190f8c4e3c0bf1c965dc.1655224415.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: isilence

fs/io_uring.c

@@ -1114,7 +1114,6 @@ static const struct io_op_def io_op_defs[] = {
 	[IORING_OP_NOP] = {
 		.audit_skip		= 1,
 		.iopoll			= 1,
-		.buffer_select		= 1,
 	},
 	[IORING_OP_READV] = {
 		.needs_file		= 1,
@@ -5269,19 +5268,7 @@ static int io_nop_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
  */
 static int io_nop(struct io_kiocb *req, unsigned int issue_flags)
 {
-	unsigned int cflags;
-	void __user *buf;
-
-	if (req->flags & REQ_F_BUFFER_SELECT) {
-		size_t len = 1;
-
-		buf = io_buffer_select(req, &len, issue_flags);
-		if (!buf)
-			return -ENOBUFS;
-	}
-
-	cflags = io_put_kbuf(req, issue_flags);
-	__io_req_complete(req, issue_flags, 0, cflags);
+	__io_req_complete(req, issue_flags, 0, 0);
 	return 0;
 }