samples, bpf: Refactor pointer error check with libbpf

Current method of checking pointer error is not user friendly.
Especially the __must_check define makes this less intuitive.

Since, libbpf has an API libbpf_get_error() which checks pointer error,
this commit refactors existing pointer error check logic with libbpf.

Signed-off-by: Daniel T. Lee <danieltimlee@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20200516040608.1377876-2-danieltimlee@gmail.com