[NETFILTER]: nfnetlink_queue: don't unregister handler of other subsystem

The queue handlers registered by ip[6]_queue.ko at initialization should
not be unregistered according to requests from userland program
using nfnetlink_queue. If we allow that, there is no way to register
the handlers of built-in ip[6]_queue again.

Signed-off-by: Yasuyuki Kozakai <yasuyuki.kozakai@toshiba.co.jp>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>