os/ObjectMap: add get_db() accessor

This is just to let us get at the underlying KeyValueDB for DBObjectMap.
It is not really better or worse than adding accessors for things like
GetEstimatedSize() to ObjectMap.

Signed-off-by: Sage Weil <sage@redhat.com>

src/os/ObjectMap.h

@@ -162,6 +162,7 @@ public:
     return ObjectMapIterator();
   }
 
+  virtual KeyValueDB *get_db() { return nullptr; }
 
   ObjectMap(CephContext* cct) : cct(cct) {}
   virtual ~ObjectMap() {}

src/os/filestore/DBObjectMap.h

@@ -58,6 +58,8 @@ class DBObjectMap : public ObjectMap {
 public:
   boost::scoped_ptr<KeyValueDB> db;
 
+  KeyValueDB *get_db() override { return db.get(); }
+
   /**
    * Serializes access to next_seq as well as the in_use set
    */