net: xfrm: use core API for updating/providing stats

Commit d3fd6548 ("net: core: add dev_sw_netstats_tx_add") has added
function "dev_sw_netstats_tx_add()" to update net device per-cpu TX
stats.

Use this function instead of own code.

While on it, remove xfrmi_get_stats64() and replace it with
dev_get_tstats64().

Signed-off-by: Lev Stipakov <lev@openvpn.net>
Reviewed-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://lore.kernel.org/r/20201113215939.147007-1-lev@openvpn.net


Signed-off-by: Jakub Kicinski <kuba@kernel.org>