nf_conntrack: Use rcu_barrier()

RCU barriers, rcu_barrier(), is inserted two places.

 In nf_conntrack_expect.c nf_conntrack_expect_fini() before the
 kmem_cache_destroy().  Firstly to make sure the callback to the
 nf_ct_expect_free_rcu() code is still around.  Secondly because I'm
 unsure about the consequence of having in flight
 nf_ct_expect_free_rcu/kmem_cache_free() calls while doing a
 kmem_cache_destroy() slab destroy.

 And in nf_conntrack_extend.c nf_ct_extend_unregister(), inorder to
 wait for completion of callbacks to __nf_ct_ext_free_rcu(), which is
 invoked by __nf_ct_ext_add().  It might be more efficient to call
 rcu_barrier() in nf_conntrack_core.c nf_conntrack_cleanup_net(), but
 thats make it more difficult to read the code (as the callback code
 in located in nf_conntrack_extend.c).

Signed-off-by: Jesper Dangaard Brouer <hawk@comx.dk>
Signed-off-by: Patrick McHardy <kaber@trash.net>