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: Eric Biggers <ebiggers3@gmail.com>
Fixes: 6e6e41c3

 ("ptr_ring: fail early if queue occupies more than KMALLOC_MAX_SIZE")
Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>