net/mlx5: optimize tunnel offload index pool

net/mlx5: optimize tunnel offload index pool

Currently, when creating the index pool, if the trunk size is not
configured, the index pool default trunk size will be 4096.

The maximum tunnel offload supported now is 256(MLX5_MAX_TUNNELS),
create the index pool with trunk size 4096 wastes the memory.

This commits changes the tunnel offload index pool trunk size to
MLX5_MAX_TUNNELS to save the memory.

Signed-off-by: Suanming Mou <suanmingm@nvidia.com>
Reviewed-by: Gregory Etelson <getelson@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>