Update curl.cpp: reduce memory cache use (#2157)

flush the file and clean the page cache when filepart download done

src/curl.cpp

@@ -703,7 +703,12 @@ size_t S3fsCurl::DownloadWriteCallback(void* ptr, size_t size, size_t nmemb, voi
     }
     pCurl->partdata.startpos += totalwrite;
     pCurl->partdata.size     -= totalwrite;
-
+
+    // flush the file and clean the page cache
+    if (pCurl->partdata.size == 0){
+        fdatasync(pCurl->partdata.fd);
+    }
+
     return totalwrite;
 }