netlink: Remove smp_read_barrier_depends() from comment

Now that smp_read_barrier_depends() has been de-emphasized, the less
said about it, the better.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Pablo Neira Ayuso <pablo@netfilter.org>
Cc: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Cc: Florian Westphal <fw@strlen.de>
Cc: <netfilter-devel@vger.kernel.org>
Cc: <coreteam@netfilter.org>