test/mempool: fix object initializer

test/mempool: fix object initializer

The function rte_pktmbuf_init() expects that the mempool private area is
large enough and was previously initialized by rte_pktmbuf_pool_init(),
which is not the case.

This causes the function rte_pktmbuf_priv_size() to return an
unpredictable value, and this value is used as a size in a memset.

Replace the mempool object initializer by my_obj_init(), which does not
have this constraint, and fits the needs for this test.

Fixes: 923ceaeac140 ("test/mempool: add unit test cases")
Cc: stable@dpdk.org
Reported-by: Wenwu Ma <wenwux.ma@intel.com>
Signed-off-by: Olivier Matz <olivier.matz@6wind.com>