net/mlx5e: xsk: Split out WQE allocation for legacy XSK RQ

Allocation of XSK frames on legacy RQ may be made more efficient with a
specialized routine that relies on certain assumptions, such as there is
only one fragment, allocation units (XSK frames) are not shared among
multiple packets. It reduces the number of branches both in the XSK code
and in the regular RQ, because with this approach there is only a single
check whether it's an XSK or regular RQ.

Signed-off-by: Maxim Mikityanskiy <maximmi@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>