[NETFILTER]: nf_log: use rcu_assign_pointer for RCU protected pointer

Also replace synchronize_net() calls by synchronize_rcu() since the
RCU protected data is also used for sysfs.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>