aio: remove a pointless assignment

the value is never used after that point
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/aio.c

@@ -1447,8 +1447,6 @@ static ssize_t aio_run_iocb(struct kiocb *req, unsigned opcode,
 			return ret;
 		}
 
-		len = ret;
-
 		if (rw == WRITE)
 			file_start_write(file);