net/mlx5e: XDP, Use multiple single-entry objects in xdpi_fifo

Here we fix the current wi->num_pkts abuse, as it was used to indicate
multiple xdpi entries in the xdpi_fifo.

Instead, reduce mlx5e_xdp_info to the size of a single field, making it
a union of unions. Per packet, use as many instances as needed to
provide the information needed at the time of completion.

The sequence of xdpi instances pushed is well defined, derived by the
xmit_mode.

Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>