netfilter: nf_conntrack: speed up reads from nf_conntrack proc file

Dumping all conntrack entries via proc interface can take hours due to
linear search to skip entries dumped so far in each cycle.

Apply same strategy used to speed up ipvs proc reading done in
commit 178883fd ("ipvs: speed up reads from ip_vs_conn proc file")
to nf_conntrack.

Note that the ctnetlink interface doesn't suffer from this problem, but
many scripts depend on the nf_conntrack proc interface.

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