samples/bpf: Switch over to libbpf

Now that libbpf under tools/lib/bpf/* is synced with the version from
samples/bpf, we can get rid most of the libbpf library here.

Committer notes:

Tested it the same way as the previous patch in this series.

Signed-off-by: Joe Stringer <joe@ovn.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexei Starovoitov <ast@fb.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Wang Nan <wangnan0@huawei.com>
Cc: netdev@vger.kernel.org
Link: http://lkml.kernel.org/r/20161214224342.12858-3-joe@ovn.org
[ Use -I$(srctree)/tools/lib/ to support out of source code tree builds, as noticed by Wang Nan ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>