perf tpebs: Fix concurrent stop races and PID reuse hazards in tpebs_stop

perf tpebs: Fix concurrent stop races and PID reuse hazards in tpebs_stop

Parallel verbose test execution can trigger a race condition in tpebs_stop
if called concurrently or when PID reuse occurs, causing finish_command()
to block or reap the wrong process.

Introduce a `tpebs_stopping` flag inside intel-tpebs.c to prevent
redundant stop execution paths, and safely restore the `cmd.pid`
temporarily only during `finish_command()` to ensure it is properly reaped,
while preventing other threads from referencing it.

Assisted-by: Gemini-CLI:Google Gemini 3
Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>