selftests/bpf: Fix crash in core_reloc when bpftool btfgen fails

Avoid unnecessary goto cleanup, as there is nothing to clean up.

Signed-off-by: Yucong Sun <fallentree@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220217180210.2981502-1-fallentree@fb.com