test/ring: fix dequeued object checks

test/ring: fix dequeued object checks

When using memcmp function to check data, the third param should be the
size of all elements, rather than the number of the elements.

Fixes: a9fe152363e2 ("test/ring: add custom element size functional tests")
Cc: stable@dpdk.org
Signed-off-by: Feifei Wang <feifei.wang2@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Reviewed-by: Phil Yang <phil.yang@arm.com>
Reviewed-by: Dharmik Thakkar <dharmik.thakkar@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>