netfilter: nf_conntrack: fix count leak in error path of __nf_conntrack_alloc

We have to decrement the conntrack counter if we fail to access the
zone extension.

Reported-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>