Don't Use Partial Datagrams with USO/GSO

[nibanks]

Describe the bug

Today, MsQuic generally fills every last byte when building USO/GSO buffers to be segmented (by stack or HW) and sent out. Some HW doesn't support a smaller trailing packet, which actually can hurt performance.

Affected OS

• Windows
• Linux
• macOS
• Other (specify below)

Additional OS information

No response

MsQuic version

main

Steps taken to reproduce bug

Run secnetperf on a platform/machine that supports USO/GSO.
Observe that it's generally sending one small packet per batch.

Expected behavior

Small packets

Actual outcome

There shouldn't be small packets unless it because application limited.

Additional details

No response

[mtfriesen]

@nibanks knocking this down to P2, since VMs in practice do not support hardware USO yet, so this would only benefit bare metal customers. Still aiming to get this done in 2502.