[NETFILTER]: ctnetlink: simplify the code to dump the conntrack table

Merge the bits to dump the conntrack table and the ones to dump and
zero counters in a single piece of code. This patch does not change
the default behaviour if accounting is not enabled.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>