net: add DEV_STATS_READ() helper

Companion of DEV_STATS_INC() & DEV_STATS_ADD().

This is going to be used in the series.

Use it in macsec_get_stats64().

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>