dup3(): put an assert and comment in case newfd is already opened.

The old code didn't really close newfd properly, put an assert
and a comment instead for now.

fs/vfs/main.c

@@ -808,21 +808,16 @@ int dup3(int oldfd, int newfd, int flags)
 	if (error)
 		goto out_errno;
 
+	/* FIXME: Should atomically close newfd or report error */
 	error = fget(newfd, &org);
-	/* ignore the error, newfd might not have an open file */
+	assert(error == 0);
 
 	error = fdset(newfd, fp);
 	if (error) {
 		fdrop(fp);
-		if (org)
-			fdrop(org);
 		goto out_errno;
 	}
 
-	/* Close previous file if it's opened. */
-	if (org)
-		fdrop(org);
-
 	fdrop(fp);
 	return newfd;