fix: off by one error in gsoSplit on linux

[encodeous]

WireGuard upstream seems to have an off-by-one error in the gsoSplit function.

A countercase would be if our outBuffs' size is 1.

if i == len(outBuffs) {
	return i - 1, ErrTooManySegments
}

Would return 0, while the fn is supposed to return "the number of buffers populated".

[Copilot]

Pull request overview

Fixes an off-by-one bug in the Linux gsoSplit implementation so that, when the output buffer slice is exhausted, the function returns the correct “number of buffers populated” value (matching its documented contract). This aligns ErrTooManySegments behavior with how callers can safely use the returned count.

Changes:

• Adjust gsoSplit overflow return value from i-1 to i to correctly report populated buffer count.
• Add a regression test to validate the overflow behavior returns len(out) populated buffers alongside ErrTooManySegments.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
polyamide/tun/offload_linux.go	Fixes overflow count returned by gsoSplit to match the “buffers populated” contract.
polyamide/tun/offload_linux_test.go	Adds a targeted regression test ensuring overflow returns the populated count and ErrTooManySegments.

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