Fix maxinfo hang

dcb_count_by_usage did not iterate the list properly and would get stuck on the
first inactive DCB. Since this function is only called by maxinfo, it would be
the only one to get stuck.

server/core/dcb.c

@@ -3007,8 +3007,8 @@ dcb_count_by_usage(DCB_USAGE usage)
                 rval++;
                 break;
             }
-        dcb = dcb->next;
         }
+        dcb = dcb->next;
     }
     spinlock_release(&dcbspin);
     return rval;