[NETFILTER]: nf_conntrack: less hairy ifdefs around proc and sysctl

Patch splits creation of /proc/net/nf_conntrack, /proc/net/stat/nf_conntrack
and net.netfilter hierarchy into their own functions with dummy ones
if PROC_FS or SYSCTL is not set. Also, remove dead "ret = 0" write
while I'm at it.

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