net/mlx5e: SHAMPO, Specialize mlx5e_fill_skb_data()

mlx5e_fill_skb_data() used to have multiple callers. But after the XDP
multibuf refactoring from commit 2cb0e27d ("net/mlx5e: RX, Prepare
non-linear striding RQ for XDP multi-buffer support") the SHAMPO code
path is the only caller.

Take advantage of this and specialize the function:
- Drop the redundant check.
- Assume that data_bcnt is > 0. This is needed in a downstream patch.

Rename the function as well to make things clear.

Signed-off-by: Dragos Tatulea <dtatulea@nvidia.com>
Suggested-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://lore.kernel.org/r/20240603212219.1037656-8-tariqt@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>