net: add DEV_STATS_READ() helper

[ Upstream commit 0b068c71 ]

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>
Stable-dep-of: ff672b9f ("ipvlan: properly track tx_errors")
Signed-off-by: Sasha Levin <sashal@kernel.org>