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

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

The mlx5e_alloc_unit union is conveniently used to store arrays of
pointers to struct page or struct xdp_buff (for xsk). The union is
currently expected to have the size of a pointer for xsk batch
allocations to work. This is conveniet for the current state of the
code but makes it impossible to add a structure of a different size
to the alloc unit.

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

This change removes the usage of mlx5e_alloc_unit union for legacy rq:

- A union of arrays is introduced (mlx5e_alloc_units) to replace the
  array of unions to allow structures of different sizes.

- Each fragment has a pointer to a unit in the mlx5e_alloc_units array.

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