net/mlx5e: XSK, Exclude tailroom from non-linear SKBs memory calculations

Packet data buffers lack reserved headroom or tailroom,
and SKBs are allocated on a side memory when needed.

Exclude the tailroom from the SKB size calculations.

Signed-off-by: Carolina Jubran <cjubran@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>