[NETFILTER]: Use the ctl paths instead of hand-made analogue

The conntracks subsystem has a similar infrastructure
to maintain ctl_paths, but since we already have it
on the generic level, I think it's OK to switch to
using it.

So, basically, this patch just replaces the ctl_table-s
with ctl_path-s, nf_register_sysctl_table with
register_sysctl_paths() and removes no longer needed code.

After this the net/netfilter/nf_sysctl.c file contains
the paths only.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>