sd-journal: fix a copy-paste error

Dereference the Directory pointer in the debug message instead of the
(probably already freed) JournalFile one.

src/libsystemd/sd-journal/sd-journal.c

@@ -3048,7 +3048,7 @@ static void process_q_overflow(sd_journal *j) {
                 if (m->is_root) /* Never GC root directories */
                         continue;
 
-                log_debug("Directory '%s' hasn't been seen in this enumeration, removing.", f->path);
+                log_debug("Directory '%s' hasn't been seen in this enumeration, removing.", m->path);
                 directory_free(m);
         }