perf tools: Fix reading new topology attribute "core_cpus"

Check if access("devices/system/cpu/cpu%d/topology/core_cpus", F_OK)
fails, which will happen unless the current directory is "/sys".

Simply try to read this file first.

Fixes: 0ccdb840 ("perf tools: Apply new CPU topology sysfs attributes")
Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Dmitry Monakhov <dmtrmonakhov@yandex-team.ru>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/158817718710.747528.11009278875028211991.stgit@buzzSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/smt.c

@@ -24,13 +24,13 @@ int smt_on(void)
 
 		snprintf(fn, sizeof fn,
 			"devices/system/cpu/cpu%d/topology/core_cpus", cpu);
-		if (access(fn, F_OK) == -1) {
+		if (sysfs__read_str(fn, &str, &strlen) < 0) {
 			snprintf(fn, sizeof fn,
 				"devices/system/cpu/cpu%d/topology/thread_siblings",
 				cpu);
+			if (sysfs__read_str(fn, &str, &strlen) < 0)
+				continue;
 		}
-		if (sysfs__read_str(fn, &str, &strlen) < 0)
-			continue;
 		/* Entry is hex, but does not have 0x, so need custom parser */
 		siblings = strtoull(str, NULL, 16);
 		free(str);