Merge branch 'libbpf-include-path'

Toke Høiland-Jørgensen says:

====================
We are currently being somewhat inconsistent with the libbpf include paths,
which makes it difficult to move files from the kernel into an external
libbpf-using project without adjusting include paths.

Having the bpf/ subdir of $INCLUDEDIR in the include path has never been a
requirement for building against libbpf before, and indeed the libbpf pkg-config
file doesn't include it. So let's make all libbpf includes across the kernel
tree use the bpf/ prefix in their includes. Since bpftool skeleton generation
emits code with a libbpf include, this also ensures that those can be used in
existing external projects using the regular pkg-config include path.

This turns out to be a somewhat invasive change in the number of files touched;
however, the actual changes to files are fairly trivial (most of them are simply
made with 'sed'). The series is split to make the change for one tool subdir at
a time, while trying not to break the build along the way. It is structured like
this:

- Patch 1-3: Trivial fixes to Makefiles for issues I discovered while changing
  the include paths.

- Patch 4-8: Change the include directives to use the bpf/ prefix, and updates
  Makefiles to make sure tools/lib/ is part of the include path, but without
  removing tools/lib/bpf

- Patch 9-11: Remove tools/lib/bpf from include paths to make sure we don't
  inadvertently re-introduce includes without the bpf/ prefix.

Changelog:

v5:
  - Combine the libbpf build rules in selftests Makefile (using Andrii's
    suggestion for a make rule).
  - Re-use self-tests libbpf build for runqslower (new patch 10)
  - Formatting fixes

v4:
  - Move runqslower error on missing BTF into make rule
  - Make sure we don't always force a rebuild selftests
  - Rebase on latest bpf-next (dropping patch 11)

v3:
  - Don't add the kernel build dir to the runqslower Makefile, pass it in from
    selftests instead.
  - Use libbpf's 'make install_headers' in selftests instead of trying to
    generate bpf_helper_defs.h in-place (to also work on read-only filesystems).
  - Use a scratch builddir for both libbpf and bpftool when building in selftests.
  - Revert bpf_helpers.h to quoted include instead of angled include with a bpf/
    prefix.
  - Fix a few style nits from Andrii

v2:
  - Do a full cleanup of libbpf includes instead of just changing the
    bpf_helper_defs.h include.
====================

Acked-by: Andrii Nakryiko <andriin@fb.com>
Tested-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>