net/mlx5e: SHAMPO, Coalesce skb fragments to page size

When doing hardware GRO (SHAMPO), the driver puts each data payload of a
packet from the wire into one skb fragment. TCP Zero-Copy expects page
sized skb fragments to be able to do it's page-flipping magic. With the
current way of arranging fragments by the driver, only specific MTUs
(page sized multiple + header size) will yield such page sized fragments
in a high percentage.

This change improves payload arrangement in the skb for hardware GRO by
coalescing payloads into a single skb fragment when possible.

To demonstrate the fix, running tcp_mmap with a MTU of 1500 yields:
- Before:  0 % bytes mmap'ed
- After : 81 % bytes mmap'ed

More importantly, coalescing considerably improves the HW GRO performance.
Here are the results for a iperf3 bandwidth benchmark:
+---------+--------+--------+------------------------+-----------+
| streams | SW GRO | HW GRO | HW GRO with coalescing | Unit      |
|---------+--------+--------+------------------------+-----------|
| 1       | 36     | 42     | 57                     | Gbits/sec |
| 4       | 34     | 39     | 50                     | Gbits/sec |
| 8       | 31     | 35     | 43                     | Gbits/sec |
+---------+--------+--------+------------------------+-----------+

Benchmark details:
VM based setup
CPU: Intel(R) Xeon(R) Platinum 8380 CPU, 24 cores
NIC: ConnectX-7 100GbE
iperf3 and irq running on same CPU over a single receive queue

Signed-off-by: Dragos Tatulea <dtatulea@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://lore.kernel.org/r/20240603212219.1037656-15-tariqt@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>