ptr_ring: prevent integer overflow when calculating size

Switch to use dividing to prevent integer overflow when size is too
big to calculate allocation size properly.
Reported-by: NEric Biggers <ebiggers3@gmail.com>
Fixes: 6e6e41c3 ("ptr_ring: fail early if queue occupies more than KMALLOC_MAX_SIZE")
Signed-off-by: NJason Wang <jasowang@redhat.com>
Acked-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/ptr_ring.h

@@ -469,7 +469,7 @@ static inline int ptr_ring_consume_batched_bh(struct ptr_ring *r,
  */
 static inline void **__ptr_ring_init_queue_alloc(unsigned int size, gfp_t gfp)
 {
-	if (size * sizeof(void *) > KMALLOC_MAX_SIZE)
+	if (size > KMALLOC_MAX_SIZE / sizeof(void *))
 		return NULL;
 	return kvmalloc_array(size, sizeof(void *), gfp | __GFP_ZERO);
 }