rtnetlink: Update now-misleading smp_read_barrier_depends() comment

Now that READ_ONCE() implies smp_read_barrier_depends(), update the
rtnl_dereference() header comment accordingly.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Vladislav Yasevich <vyasevic@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Vlad Yasevich <vyasevich@gmail.com>