perf pmu arm64: Fix reading the PMU cpu slots in sysfs

perf pmu arm64: Fix reading the PMU cpu slots in sysfs

Commit f8ad6018ce3c065a ("perf pmu: Remove duplication around
EVENT_SOURCE_DEVICE_PATH") uses sysfs__read_ull() to read a full sysfs
path, which will never succeeds as it already comes with the sysfs mount
point in it, which sysfs__read_ull() will add again.

Fix it by reading the file using filename__read_ull(), that will not add
the sysfs mount point.

Fixes: f8ad6018ce3c065a ("perf pmu: Remove duplication around EVENT_SOURCE_DEVICE_PATH")
Signed-off-by: Haixin Yu <yuhaixin.yhx@linux.alibaba.com>
Tested-by: Jing Zhang <renyu.zj@linux.alibaba.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@arm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: John Garry <john.g.garry@oracle.com>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mike Leach <mike.leach@linaro.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Link: https://lore.kernel.org/r/ZL4G7rWXkfv-Ectq@B-Q60VQ05P-2326.local
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>