selftest: fix set-attach test

After libbpf commit 3fd6eebb2d4 ("libbpf: disassociate section handler
on explicit bpf_program__set_type() call"):

 If user explicitly overrides programs's type with
 bpf_program__set_type() API call, we need to disassociate whatever
 SEC_DEF handler libbpf determined initially based on program's SEC()
 definition, as it's not goind to be valid anymore and could lead to
 crashes and/or confusing failures.

The eBPF fentry program macro should only have the "fentry" keyword.

selftest/set-attach/main.bpf.c

@@ -11,7 +11,7 @@ struct {
 } events SEC(".maps");
 long ringbuffer_flags = 0;
 
-SEC("fentry/FUNC")
+SEC("fentry")
 int BPF_PROG(foobar)
 {
     int *process;

selftest/set-attach/main.go

@@ -29,6 +29,7 @@ func main() {
 
 	prog.SetProgramType(bpf.BPFProgTypeTracing)
 	prog.SetAttachType(bpf.BPFAttachTypeTraceFentry)
+
 	err = prog.SetAttachTarget(0, "__x64_sys_mmap")
 	if err != nil {
 		fmt.Fprintln(os.Stderr, err)