tools/trace: fix -M/--max-events exceeding the specified limit

[Gbell26]

Description

When the event count reaches the -M limit, Probe.done is set to True, but remaining events in the current poll continue to be processed because the callback never checks the flag at entry.

This causes -M to be unpredictable and occasionally print many more events than max_events

ex:
# sudo /usr/share/bcc/tools/trace -M 50 'c:malloc "size = %d", arg1' | wc -l 491
Add an early return at the top of print_event when Probe.done is already set, so events beyond the limit are not printed.

Why this approach

AFAICT this is the only point to interrupt inbetween individual events and stopping the entirety of the returned events from being printed

Checklist

• [ x] Commit prefix matches changed area (e.g., tools/toolname:, libbpf-tools/toolname:, src/cc:, docs:, build:, tests/python:)
• [x ] Commit body explains why this change is needed

For new tools only

• Explains why this tool is needed and what existing tools cannot cover this use case
• Includes at least one real production use case
• Man page (man/man8/) with an OVERHEAD section
• Example output file (*_example.txt)
• README.md entry added
• Smoke test added to tests/python/test_tools_smoke.py

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