Add back mnesia dir reset/restore

Use a more descriptive variable name, which explains why this can't be
actually squashed into a single call to rabbit_mnesia:dir/0.

Author: Alexey Lebedeff

src/rabbit_mnesia.erl

@@ -840,11 +840,12 @@ with_running_or_clean_mnesia(Fun) ->
     case IsMnesiaRunning of
         true  -> Fun();
         false ->
+            SavedMnesiaDir = dir(),
             application:unset_env(mnesia, dir),
             mnesia:start(),
             Result = Fun(),
             application:stop(mnesia),
-            application:set_env(mnesia, dir, dir()),
+            application:set_env(mnesia, dir, SavedMnesiaDir),
             Result
     end.