net/mlx5e: Refactor xmit functions

net/mlx5e: Refactor xmit functions

A huge function mlx5e_sq_xmit was split into several to achieve multiple
goals:

1. Reuse the code in IPoIB.

2. Better intergrate with TLS, IPSEC, GENEVE and checksum offloads. Now
it's possible to reserve space in the WQ before running eseg-based
offloads, so:

2.1. It's not needed to copy cseg and eseg after mlx5e_fill_sq_frag_edge
anymore.

2.2. mlx5e_txqsq_get_next_pi will be used instead of the legacy
mlx5e_fill_sq_frag_edge for better code maintainability and reuse.

3. Prepare for the upcoming TX MPWQE for SKBs. It will intervene after
mlx5e_sq_calc_wqe_attr to check if it's possible to use MPWQE, and the
code flow will split into two paths: MPWQE and non-MPWQE.

Two high-level functions are provided to send packets:

* mlx5e_xmit is called by the networking stack, runs offloads and sends
the packet. In one of the following patches, MPWQE support will be added
to this flow.

* mlx5e_sq_xmit_simple is called by the TLS offload, runs only the
checksum offload and sends the packet.

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: 16.86 Mpps (±0.15 Mpps) -> 16.95 Mpps (±0.15 Mpps)
  Instructions per packet: 434 -> 429
  Cycles per packet: 158 -> 160
  Instructions per cycle: 2.75 -> 2.69

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>