[NETFILTER]: ip{6}_queue: prevent unregistration race with nfnetlink_queue

Since nfnetlink_queue can override ip{6}_queue as queue handlers, we
can no longer blindly unregister whoever is registered for PF_INET[6],
but only unregister ourselves.

Signed-off-by: Harald Welte <laforge@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>