Merge branch 'fix-samples'

Jakub Kicinski says:

====================
Following patches address build issues after recent move to libbpf.
For out-of-tree builds we would see the following error:

gcc: error: samples/bpf/../../tools/lib/bpf/libbpf.a: No such file or directory

libbpf build system is now always invoked explicitly rather than
relying on building single objects most of the time.  We need to
resolve the friction between Kbuild and tools/ build system.

Mini-library called libbpf.h in samples is renamed to bpf_insn.h,
using linux/filter.h seems not completely trivial since some samples
get upset when order on include search path in changed.  We do have
to rename libbpf.h, however, because otherwise it's hard to reliably
get to libbpf's header in out-of-tree builds.

v2:
 - fix the build error harder (patch 3);
 - add patch 5 (make clang less noisy).
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>