netfilter: nf_conntrack: do not skip entries in /proc/net/nf_conntrack

ct_seq_show() has an opportunistic garbage collector :

if (nf_ct_should_gc(ct)) {
    nf_ct_kill(ct);
    goto release;
}

So if one nf_conn is killed there, next time ct_get_next() runs,
we skip the following item in the bucket, even if it should have
been displayed if gc did not take place.

We can decrement st->skip_elems to tell ct_get_next() one of the items
was removed from the chain.

Fixes: 58e207e4 ("netfilter: evict stale entries when user reads /proc/net/nf_conntrack")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Florian Westphal <fw@strlen.de>