net/mlx5e: RX, Rename xdp_xmit_bitmap to a more generic name

The xdp_xmit_bitmap currently serves only one purpose: to avoid
releasing pages that are still in use due to XDP TX.

A following patch will use this bitmap in a slightly different context
but for the same purpose. So rename the bitmap to a more generic name
that reflects the purpose not the context.

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