Annotate libbpf API's with _Must_inspect_result_

The libbpf API's use an int to signal success or failure. We should add _Must_inspect_result_ to help catch bugs where callers potentially ignore failures.

Locate the libbpf API declarations and any existing _Must_inspect_result_ annotation conventions first; the issue does not name specific files or tests. Identify the in-scope APIs that use int for success or failure, then verify that the annotations are applied consistently and that the project still builds and tests successfully.

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