ptnetmap: ptnetmap_tx_handler: don't use NM_ACCESS_ONCE for hwtail
ptnetmap: ptnetmap_tx_handler: don't use NM_ACCESS_ONCE for hwtail

hwtail for TX krings is modified only by the txsync of the parent
adapter, which is only called by ptnetmap_tx_handler().

luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap