io_uring/fs: remove sqe->rw_flags checking from LINKAT

This is unionized with the actual link flags, so they can of course be
set and they will be evaluated further down. If not we fail any LINKAT
that has to set option flags.

Fixes: cf30da9 ("io_uring: add support for IORING_OP_LINKAT")
Cc: stable@vger.kernel.org
Reported-by: Thomas Leonard <talex5@gmail.com>
Link: https://github.com/axboe/liburing/issues/955
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/fs.c

@@ -243,7 +243,7 @@ int io_linkat_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 	struct io_link *lnk = io_kiocb_to_cmd(req, struct io_link);
 	const char __user *oldf, *newf;
 
-	if (sqe->rw_flags || sqe->buf_index || sqe->splice_fd_in)
+	if (sqe->buf_index || sqe->splice_fd_in)
 		return -EINVAL;
 	if (unlikely(req->flags & REQ_F_FIXED_FILE))
 		return -EBADF;