netfilter: nf_conntrack: fix sysctl memory leak

Message in log because sysctl table was not empty at netns exit
 WARNING: at net/sysctl_net.c:84 sysctl_net_exit+0x2a/0x2c()

Instrumenting showed that the nf_conntrack_timestamp was the entry
that was being created but not cleared.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>