netfilter: netns nf_conntrack: per-netns net.netfilter.nf_conntrack_count sysctl

Note, sysctl table is always duplicated, this is simpler and less
special-cased.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>