mds: MDCache request cleanup handles potential null mdr

In cases where there is a single element in a batch_op_map,new_batch_head
is a nullptr, when this is retried at Finisher we'd hit one of the asserts when
dereferencing

Fixes: https://tracker.ceph.com/issues/70769
Signed-off-by: Abhishek Lekshmanan <[email protected]>

Author: theanalyst

src/mds/MDCache.cc

@@ -10090,8 +10090,9 @@ void MDCache::request_cleanup(const MDRequestRef& mdr)
     auto new_batch_head = it->second->find_new_head();
     if (!new_batch_head) {
       mdr->batch_op_map->erase(it);
+    } else {
+      mds->finisher->queue(new C_MDS_RetryRequest(this, new_batch_head));
     }
-    mds->finisher->queue(new C_MDS_RetryRequest(this, new_batch_head));
   }
 
   if (mdr->has_more()) {