crypto/null: use ring size function
crypto/null: use ring size function

Rather than reading the size directly from the ring structure,
use the dedicated function for that purpose.

Signed-off-by: Bruce Richardson <66b9e8e8fbd5e9f41f91b0af050876182ce9d0e4@intel.com>
Acked-by: Declan Doherty <b4388acd850cd8caad5000a112795063f04807ac@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk