Ensure to access RecoveryState#fileDetails under lock

Closes #43840

server/src/main/java/org/elasticsearch/indices/recovery/RecoveryState.java

@@ -956,7 +956,7 @@ public class RecoveryState implements ToXContentFragment, Streamable, Writeable
             }
         }
 
-        public File getFileDetails(String dest) {
+        public synchronized File getFileDetails(String dest) {
             return fileDetails.get(dest);
         }
     }