test/mempool: fix test on FreeBSD

test/mempool: fix test on FreeBSD

FreeBSD EAL does not implement rte_mem_virt2iova() causing an error:

    EAL: Test assert test_mempool_flag_non_io_unset_when_populated_with_valid_iova
    line 781 failed: Cannot get IOVA
    test failed at test_mempool():1030
    Test Failed

Change unit test to use rte_memzone_reserve() to allocate memory,
which allows to obtain IOVA directly.

Bugzilla ID: 863
Fixes: 11541c5c81dd ("mempool: add non-IO flag")
Reported-by: Yu Jiang <yux.jiang@intel.com>
Signed-off-by: Dmitry Kozlyuk <dkozlyuk@nvidia.com>