io_tester: ensure that file object is valid when closing it

In the case of unlink workloads they create and fill
files and close them right away.

This change ensures that the file object is valid until
close() finishes.

Signed-off-by: Patryk Wrobel <[email protected]>

Closes scylladb#2307

apps/io_tester/io_tester.cc

@@ -784,7 +784,9 @@ class unlink_class_data : public class_data {
             options.append_is_unlikely = true;
 
             return create_and_fill_file(fname, fsize, flags, options).then([](std::pair<file, uint64_t> p) {
-                return p.first.close();
+                return do_with(std::move(p.first), [] (auto& f) {
+                    return f.close();
+                });
             });
         });
     }