perf stat: Fix error check for bpf_program__attach

It seems the bpf_program__attach() returns a negative error code instead
of a NULL pointer in case of error.

Fixes: 7fac83aa ("perf stat: Introduce 'bperf' to share hardware PMCs with BPF")
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Song Liu <songliubraving@fb.com>
Link: http://lore.kernel.org/lkml/20210527220052.1657578-1-namhyung@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/bpf_counter.c

@@ -451,10 +451,10 @@ static int bperf_reload_leader_program(struct evsel *evsel, int attr_map_fd,
 		goto out;
 	}
 
-	err = -1;
 	link = bpf_program__attach(skel->progs.on_switch);
-	if (!link) {
+	if (IS_ERR(link)) {
 		pr_err("Failed to attach leader program\n");
+		err = PTR_ERR(link);
 		goto out;
 	}