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: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200514053207.1298479-1-yhs@fb.com