ptr_ring: linked list fallback

So pointer rings work fine, but they have a problem: make them too small
and not enough entries fit.  Make them too large and you start flushing
your cache and running out of memory.

This is a new idea of mine: a ring backed by a linked list. Once you run
out of ring entries, instead of a drop you fall back on a list with a
common lock.

Should work well for the case where the ring is typically sized
correctly, but will help address the fact that some user try to set e.g.
tx queue length to 1000000.

In other words, the idea is that if a user sets a really huge TX queue
length, we allocate a ptr_ring which is smaller, and use the backup
linked list when necessary to provide the requested TX queue length
legitimately.

My hope this will move us closer to direction where e.g. fw codel can
use ptr rings without locking at all.  The API is still very rough, and
I really need to take a hard look at lock nesting.

Compiled only, sending for early feedback/flames.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>