remove length in ShmSocket ctor, not used.

lichao

2021-06-01 f7f8fbc85c48b003d1e3094a94a3528001977977

 utest/robust_test.cpp

@@ -75,7 +75,7 @@
   int64_t d;
   BOOST_CHECK(tmp.pop(d));

   ShmObject<Rcb> rcb(shm, "test_rcb");
   ShmObject<Rcb> rcb(shm, "test_rcb", eOpenOrCreate);
   bool try_more = true;

   auto Writer = [&]() {