[NETFILTER]: Properly use RCU in nf_ct_attach

Use rcu_assign_pointer/rcu_dereference for ip_ct_attach pointer instead
of self-made RCU and use rcu_read_lock to make sure the conntrack module
doesn't disappear below us while calling it, since this function can be
called from outside the netfilter hooks.

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