samples/bpf: Partially fixes the bpf.o build

samples/bpf: Partially fixes the bpf.o build

[ Upstream commit c25ef6a5e62fa212d298ce24995ce239f29b5f96 ]

Do not build lib/bpf/bpf.o with this Makefile but use the one from the
library directory.  This avoid making a buggy bpf.o file (e.g. missing
symbols).

This patch is useful if some code (e.g. Landlock tests) needs both the
bpf.o (from tools/lib/bpf) and the bpf_load.o (from samples/bpf).

Signed-off-by: Mickaël Salaün <mic@digikod.net>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>