net/mlx5e: Convert struct mlx5e_alloc_unit to a union

struct mlx5e_alloc_unit consists of a single union. Convert it to a
union itself to simplify casting it to struct xdp_buff *, which will be
used to implement XSK batching on striding RQ.

Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>