ofproto-dpif-xlate: Allow direct destroy of previous config.

Before this commit, the ofproto-dpif-xlate module uses ovs-rcu to
postpone the destroy of previous configuration.  However, the delayed
close of object like 'struct netdev' could cause failure in immediate
re-add or reconfigure of the same device.

To fix the above issue, this commit makes the ofproto-dpif-xlate
module call ovsrcu_synchronize(), which waits for all threads
to finish the use of reference to previous config.  Then, the
module can just directly destroy the previous config.

Reported-by: Cian Ferriter <[email protected]>
Signed-off-by: Alex Wang <[email protected]>
Acked-by: Ben Pfaff <[email protected]>

ofproto/ofproto-dpif-xlate.c

@@ -672,8 +672,8 @@ xlate_txn_commit(void)
     struct xlate_cfg *xcfg = ovsrcu_get(struct xlate_cfg *, &xcfgp);
 
     ovsrcu_set(&xcfgp, new_xcfg);
-    ovsrcu_postpone(xlate_xcfg_free, xcfg);
-
+    ovsrcu_synchronize();
+    xlate_xcfg_free(xcfg);
     new_xcfg = NULL;
 }