net/mlx5e: xsk: Remove mlx5e_xsk_page_alloc_pool

mlx5e_xsk_page_alloc_pool became a thin wrapper around xsk_buff_alloc.
Drop it and call xsk_buff_alloc directly.

Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>