netfilter: nft_socket: make cgroup match work in input too

cgroupv2 helper function ignores the already-looked up sk
and uses skb->sk instead.

Just pass sk from the calling function instead; this will
make cgroup matching work for udp and tcp in input even when
edemux did not set skb->sk already.

Fixes: e0bb96db ("netfilter: nft_socket: add support for cgroupsv2")
Signed-off-by: Florian Westphal <fw@strlen.de>
Tested-by: Topi Miettinen <toiwoton@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>