netfilter: add and use jump label for xt_tee

Don't bother testing if we need to switch to alternate stack
unless TEE target is used.

Suggested-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>