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: NPaul 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>

include/linux/rtnetlink.h

@@ -70,8 +70,7 @@ static inline bool lockdep_rtnl_is_held(void)
  * @p: The pointer to read, prior to dereferencing
  *
  * Return the value of the specified RCU-protected pointer, but omit
- * both the smp_read_barrier_depends() and the READ_ONCE(), because
- * caller holds RTNL.
+ * the READ_ONCE(), because caller holds RTNL.
  */
 #define rtnl_dereference(p)					\
 	rcu_dereference_protected(p, lockdep_rtnl_is_held())