xfrm: use new function dev_fetch_sw_netstats

Simplify the code by using new function dev_fetch_sw_netstats().

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://lore.kernel.org/r/a6b816f4-bbf2-9db0-d59a-7e4e9cc808fe@gmail.com


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