perf hwmon_pmu: Hold path rather than fd

Hold the path to the hwmon_pmu rather than the file descriptor. The
file descriptor is somewhat problematic in that it reflects the
directory state when opened, something that may vary in testing. Using
a path simplifies testing and to some extent cleanup as the hwmon_pmu
is owned by the pmus list and intentionally global and leaked when
perf terminates, the file descriptor being left open looks like a
leak.

Signed-off-by: Ian Rogers <irogers@google.com>
Link: https://lore.kernel.org/r/20250624190326.2038704-4-irogers@google.com


Signed-off-by: Namhyung Kim <namhyung@kernel.org>