ptr_ring: fix up after recent ptr_ring changes

[ Upstream commit 5790eabc6e7c3ce2d6ca2e3bbf4de467ce2b64b3 ]

Add more stubs to make it build.

Fixes: 81fbfe8a ("ptr_ring: use kmalloc_array()")
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>