ip6_tunnel: use dev_sw_netstats_rx_add()

[ Upstream commit afd2051b ]

We have a convenient helper, let's use it.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Stable-dep-of: 8d975c15 ("ip6_tunnel: make sure to pull inner header in __ip6_tnl_rcv()")
Signed-off-by: Sasha Levin <sashal@kernel.org>