net/mlx5e: RX, Remove alloc unit layout constraint for striding rq

This change removes the usage of mlx5e_alloc_unit union for
striding rq. The change is more straightforward than legacy rq as
the alloc units union is already in place.

This patch only moves things around: instead of an array of unions make
it a union of arrays. This has the effect that each mlx5e_mpw_info will
allocate the largest possible size of the array member. For xsk this
means that the array of xdp_buff pointers for the wqe will still be
contiguous, but there will be some extra unused bytes at the end of the
array.

As further patch in the series will add the mlx5e_frag_page struct for
which the described size constraint will no longer hold.

Signed-off-by: Dragos Tatulea <dtatulea@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>