Fix sendmmsg wrapper

Summary: Fix sendmmsg wrapper

Reviewed By: yfeldblum

Differential Revision: D21513176

fbshipit-source-id: ddc1bdd93801136994ef62fd5793da7371e9e22a

folly/net/NetOps.cpp

@@ -412,6 +412,8 @@ int sendmmsg(
       }
 
       return static_cast<int>(ret);
+    } else {
+      msgvec[i].msg_len = ret;
     }
   }