net/mlx5e: Support multiple SKBs in a TX WQE

net/mlx5e: Support multiple SKBs in a TX WQE

TX MPWQE support for SKBs is coming in one of the following patches, and
a single MPWQE can send multiple SKBs. This commit prepares the TX path
code to handle such cases:

1. An additional FIFO for SKBs is added, just like the FIFO for DMA
chunks.

2. struct mlx5e_tx_wqe_info will contain num_fifo_pkts. If a given WQE
contains only one packet, num_fifo_pkts will be zero, and the SKB will
be stored in mlx5e_tx_wqe_info, as usual. If num_fifo_pkts > 0, the SKB
pointer will be NULL, and the SKBs will be stored in the FIFO.

This change has no performance impact in TCP single stream test and
XDP_TX single stream test.

When compiled with a recent GCC, this change shows no visible
performance impact on UDP pktgen (burst 32) single stream test either:
  Packet rate: 16.95 Mpps (±0.15 Mpps) -> 16.96 Mpps (±0.12 Mpps)
  Instructions per packet: 429 -> 421
  Cycles per packet: 160 -> 156
  Instructions per cycle: 2.69 -> 2.70

CPU: Intel(R) Xeon(R) CPU E5-2680 v3 @ 2.50GHz (x86_64)
NIC: Mellanox ConnectX-6 Dx
GCC 10.2.0

Signed-off-by: Maxim Mikityanskiy <maximmi@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>