xfrm: Check inner packet family directly from skb_dst

xfrm: Check inner packet family directly from skb_dst

In the output path, xfrm_dev_offload_ok and xfrm_get_inner_ipproto
need to determine the protocol family of the inner packet (skb) before
it gets encapsulated.

In xfrm_dev_offload_ok, the code checked x->inner_mode.family. This is
unreliable because, for states handling both IPv4 and IPv6, the
relevant inner family could be either x->inner_mode.family or
x->inner_mode_iaf.family. Checking only the former can lead to a
mismatch with the actual packet being processed.

In xfrm_get_inner_ipproto, the code checked x->outer_mode.family. This
is also incorrect for tunnel mode, as the inner packet's family can be
different from the outer header's family.

At both of these call sites, the skb variable holds the original inner
packet. The most direct and reliable source of truth for its protocol
family is its destination entry. This patch fixes the issue by using
skb_dst(skb)->ops->family to ensure protocol-specific headers are only
accessed for the correct packet type.

Fixes: 91d8a53db219 ("xfrm: fix offloading of cross-family tunnels")
Fixes: 45a98ef4922d ("net/xfrm: IPsec tunnel mode fix inner_ipproto setting in sec_path")
Signed-off-by: Jianbo Liu <jianbol@nvidia.com>
Reviewed-by: Cosmin Ratiu <cratiu@nvidia.com>
Reviewed-by: Zhu Yanjun <yanjun.zhu@linux.dev>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>