net/mlx5: support zero size entry for indexed pool
net/mlx5: support zero size entry for indexed pool

To make indexed pool to be used as ID generator, this patch allows entry
size to be zero.

Signed-off-by: Xueming Li <a578f76ca98b1dec92e0d1baef814b93f3f2f3b4@nvidia.com>
Acked-by: Matan Azrad <6969f5f553ad7652679a585a85d7321babc90464@nvidia.com>

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