netfilter: nft_flow_offload: IPCB is only valid for ipv4 family

commit 69aeb538587e087bfc81dd1f465eab3558ff3158 upstream.

Guard this with a check vs. ipv4, IPCB isn't valid in ipv6 case.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>