future-util: do not "return move(local_var)"

GCC and clang warns like

warning: moving a local object in a return statement prevents copy
elision [-Wpessimizing-move]
  580 |             return std::move(f);
      |                               ^

Signed-off-by: Kefu Chai <[email protected]>

include/seastar/core/future-util.hh

@@ -577,7 +577,7 @@ future<> do_for_each(Iterator begin, Iterator end, AsyncAction action) {
             });
         }
         if (f.failed()) {
-            return std::move(f);
+            return f;
         }
     }
 }