Revert "ptr_ring: move free-space check into separate helper"

Revert "ptr_ring: move free-space check into separate helper"

This reverts commit fba362c17d9d9211fc51f272156bb84fc23bdf98.

__ptr_ring_check_produce() has no users left after reverting
commit 1d6e569b7d0c ("tun/tap & vhost-net: avoid ptr_ring tail-drop
when a qdisc is present").

Signed-off-by: Simon Schippers <simon.schippers@tu-dortmund.de>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Link: https://patch.msgid.link/20260728092240.250257-3-simon.schippers@tu-dortmund.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>