perf pmu: Add perf_pmu__{open,scan}_file_at()

These two helpers will also use openat() to reduce the overhead with
relative pathnames.  Convert other functions in pmu_lookup() to use
the new helpers.

Committer testing:

Before:

  ⬢[acme@toolbox perf-tools-next]$ perf bench internals pmu-scan
  # Running 'internals/pmu-scan' benchmark:
  Computing performance of sysfs PMU event scan for 100 times
    Average PMU scanning took: 2729.040 usec (+- 7.117 usec)
  ⬢[acme@toolbox perf-tools-next]$

After:

  ⬢[acme@toolbox perf-tools-next]$ perf bench internals pmu-scan
  # Running 'internals/pmu-scan' benchmark:
  Computing performance of sysfs PMU event scan for 100 times
    Average PMU scanning took: 2419.870 usec (+- 9.057 usec)
  ⬢[acme@toolbox perf-tools-next]$

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Ian Rogers <irogers@google.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230331202949.810326-2-namhyung@kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>