Fix the aggregation of the interface level counters `dev.vtnet.X.tx_task_rescheduled`, `dev.vtnet.X.tx_tso_offloaded`, `dev.vtnet.X.tx_csum_offloaded`, `dev.vtnet.X.rx_task_rescheduled`, `dev.vtnet.X.rx_csum_offloaded`, and `dev.vtnet.X.rx_csum_failed`. Also ensure that `dev.vtnet.X.tx_defrag_failed` only counts the number of times `m_defrag()` fails.
While there, mark `sysctl`-variables used for exporting statistics as such (`CTLFLAG_STATS`).