net: add a couple of helpers for iph tot_len

[ Upstream commit 058a8f7f ]

This patch adds three APIs to replace the iph->tot_len setting
and getting in all places where IPv4 BIG TCP packets may reach,
they will be used in the following patches.

Note that iph_totlen() will be used when iph is not in linear
data of the skb.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Stable-dep-of: b2dc32dc ("net/sched: act_ipt: add sanity checks on skb before calling target")
Signed-off-by: Sasha Levin <sashal@kernel.org>