selftests/bpf: Enforce returning 0 for fentry/fexit programs

There are a few fentry/fexit programs returning non-0.
The tests with these programs will break with the previous
patch which enfoced return-0 rules. Fix them properly.

Fixes: ac065870 ("selftests/bpf: Add BPF_PROG, BPF_KPROBE, and BPF_KRETPROBE macros")
Signed-off-by: NYonghong Song <yhs@fb.com>
Signed-off-by: NAlexei Starovoitov <ast@kernel.org>
Acked-by: NAndrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200514053207.1298479-1-yhs@fb.com

tools/testing/selftests/bpf/progs/test_overhead.c

@@ -30,13 +30,13 @@ int prog3(struct bpf_raw_tracepoint_args *ctx)
 SEC("fentry/__set_task_comm")
 int BPF_PROG(prog4, struct task_struct *tsk, const char *buf, bool exec)
 {
-	return !tsk;
+	return 0;
 }
 
 SEC("fexit/__set_task_comm")
 int BPF_PROG(prog5, struct task_struct *tsk, const char *buf, bool exec)
 {
-	return !tsk;
+	return 0;
 }
 
 char _license[] SEC("license") = "GPL";