os/bluestore: fix race condition

The empty() method also needs to be called under the protection of lock.

Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>

src/os/bluestore/BlueStore.h

@@ -378,6 +378,7 @@ class BlueStore : public ObjectStore,
     }
 
     bool empty() {
+      std::lock_guard<std::mutex> l(lock);
       return uset.empty();
     }
   };