Keeping un-unmmappable pages until they can be reused

On Linux, munmap() may fail with ENOMEM when virtual memory is too
fragmented. Working this around by just keeping such blocks for future
use.

lib/locking.h

@@ -19,6 +19,7 @@ struct lock_order {
   struct domain_generic *attrs;
   struct domain_generic *cork;
   struct domain_generic *event;
+  struct domain_generic *resource;
 };
 
 extern _Thread_local struct lock_order locking_stack;