[NETFILTER]: ip_conntrack: fix NAT helper unload races

The NAT helpr hooks are protected by RCU, but all of the
conntrack helpers test and use the global pointers instead
of copying them first using rcu_dereference()

Also replace synchronize_net() by synchronize_rcu() for clarity
since sychronizing only with packet receive processing is
insufficient to prevent races.

Signed-off-by: Patrick McHardy <kaber@trash.net>