net/mlx5e: xsk: Use xsk_buff_alloc_batch on legacy RQ

XSK provides a function to allocate frames in batches for more efficient
processing. This commit starts using this function on legacy RQ, adding
a special case for XSK. The new branch introduced basically replaces the
branch that was removed from the same place a few commits before.

A check is made that DMA sync is not needed, because the batching
allocator falls back to returning one frame when DMA sync is needed, and
this is best handled by the loop in the standard case.

Performance improvement is up to 8% in the aligned mode and up to 9% in
the unaligned mode.

Aligned mode, 2048-byte frames: 12.8 Mpps -> 13.5 Mpps
Aligned mode, 4096-byte frames: 11.5 Mpps -> 12.4 Mpps
Unaligned mode, 2048-byte frames: 12.2 Mpps -> 13.4 Mpps
Unaligned mode, 3072-byte frames: 11.6 Mpps -> 12.5 Mpps
Unaligned mode, 4096-byte frames: 11.2 Mpps -> 12.2 Mpps

CPU: Intel(R) Xeon(R) Gold 6240 CPU @ 2.60GHz

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>