MFC r347255:
Fix tx_jumbo_packets counter in mlx5en(4).

Instead of reading Ethernet RFC 2819 pXtoYoctets counters from
hardware which counts RX octets, count tx_stat_pXtoYoctets from
Ethernet extended counters which counts TX octets.

TX jumbo counters should be accumulated only after the PPCNT
counters were fetched from hardware with their latest value.

Submitted by: slavash@
Sponsored by: Mellanox Technologies