ptr_ring: disallow lockless __ptr_ring_full

Similar to bcecb4bb ("net: ptr_ring: otherwise safe empty checks can
overrun array bounds") a lockless use of __ptr_ring_full might
cause an out of bounds access.

We can fix this, but it's easier to just disallow lockless
__ptr_ring_full for now.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/ptr_ring.h

@@ -45,9 +45,10 @@ struct ptr_ring {
 };
 
 /* Note: callers invoking this in a loop must use a compiler barrier,
- * for example cpu_relax().  If ring is ever resized, callers must hold
- * producer_lock - see e.g. ptr_ring_full.  Otherwise, if callers don't hold
- * producer_lock, the next call to __ptr_ring_produce may fail.
+ * for example cpu_relax().
+ *
+ * NB: this is unlike __ptr_ring_empty in that callers must hold producer_lock:
+ * see e.g. ptr_ring_full.
  */
 static inline bool __ptr_ring_full(struct ptr_ring *r)
 {