ptr_ring: Remove now-redundant smp_read_barrier_depends()

Because READ_ONCE() now implies smp_read_barrier_depends(), the
smp_read_barrier_depends() in __ptr_ring_consume() is redundant;
this commit removes it and updates the comments.

Signed-off-by: Andrea Parri <parri.andrea@gmail.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: <linux-kernel@vger.kernel.org>
Cc: <netdev@vger.kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>