chore(fs): add debug logging for case cache registry (#9869)

Clearly something is up with it, but I still have no clue what. This
might give some clue when affected user enable debug logging.

lib/fs/casefs.go

@@ -89,9 +89,13 @@ func (r *caseFilesystemRegistry) get(fs Filesystem) Filesystem {
 			r.startCleaner.Do(func() {
 				go r.cleaner()
 			})
+			l.Debugf("Created new case cache for key %v", k)
 		}
 		r.mut.Unlock()
 	}
+	if ok {
+		l.Debugf("Reusing case cache for key %v", k)
+	}
 
 	return &caseFilesystem{
 		Filesystem: fs,