ring: fix error code when creating ring

ring: fix error code when creating ring

The error value returned by rte_ring_create_elem() should be positive
integers. However, if the rte_ring_get_memsize_elem() function fails,
a negative number is returned and is directly used as the return value.
As a result, this will cause the external call to check the return
value to fail(like called by rte_mempool_create()).

Fixes: a182620042aa ("ring: get size in memory")
Cc: stable@dpdk.org
Reported-by: Nan Zhou <zhounan14@huawei.com>
Signed-off-by: Yunjian Wang <wangyunjian@huawei.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>