ptr_ring: drop duplicated tail zeroing code

ptr_ring: drop duplicated tail zeroing code

We have some rather subtle code around zeroing tail entries, minimizing
cache bouncing.  Let's put it all in one place.

Doing this also reduces the text size slightly, e.g. for
drivers/vhost/net.o
  Before: text: 15,114 bytes
  After: text: 15,082 bytes

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Link: https://patch.msgid.link/adb9d941de4a2b619ddb2be271a9939849e70687.1758690291.git.mst@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>