netfilter: fix two recent sysctl problems

Starting with 9043476f726802f4b00c96d0c4f418dde48d1304 ("[PATCH]
sanitize proc_sysctl") we have two netfilter releated problems:

 - WARNING: at kernel/sysctl.c:1966 unregister_sysctl_table+0xcc/0x103(),
   caused by wrong order of ini/fini calls

 - net.netfilter is duplicated and has truncated set of records

Thanks to very useful guidelines from Al Viro, this patch fixes both
of them.

Signed-off-by: Krzysztof Piotr Oledzki <ole@ans.pl>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>