perf jevents: Ignore sys when determining a model directory

perf jevents: Ignore sys when determining a model directory

Existing sys directories aren't placed under a model directory like
skylake.

Placing a sys directory there causes the `is_leaf_dir` test to fail and
consequently no events or metrics are generated for the model.

Ignore sys directories in this case and update the comments to
reflect why.

This change has no affect, but when testing with a sys directory for a
model people have reported running into the no event/metric issue.

Reported-by: Stephane Eranian <eranian@google.com>
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: Jing Zhang <renyu.zj@linux.alibaba.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: John Garry <john.g.garry@oracle.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sandipan Das <sandipan.das@amd.com>
Cc: Thomas Richter <tmricht@linux.ibm.com>
Cc: Xu Yang <xu.yang_2@nxp.com>
Link: https://lore.kernel.org/r/20240904211705.915101-1-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>