Merge branch 'bpf: add support for new btf kind BTF_KIND_TAG'

Yonghong Song says:

====================

LLVM14 added support for a new C attribute ([1])
  __attribute__((btf_tag("arbitrary_str")))
This attribute will be emitted to dwarf ([2]) and pahole
will convert it to BTF. Or for bpf target, this
attribute will be emitted to BTF directly ([3], [4]).
The attribute is intended to provide additional
information for
  - struct/union type or struct/union member
  - static/global variables
  - static/global function or function parameter.

This new attribute can be used to add attributes
to kernel codes, e.g., pre- or post- conditions,
allow/deny info, or any other info in which only
the kernel is interested. Such attributes will
be processed by clang frontend and emitted to
dwarf, converting to BTF by pahole. Ultimiately
the verifier can use these information for
verification purpose.

The new attribute can also be used for bpf
programs, e.g., tagging with __user attributes
for function parameters, specifying global
function preconditions, etc. Such information
may help verifier to detect user program
bugs.

After this series, pahole dwarf->btf converter
will be enhanced to support new llvm tag
for btf_tag attribute. With pahole support,
we will then try to add a few real use case,
e.g., __user/__rcu tagging, allow/deny list,
some kernel function precondition, etc,
in the kernel.

In the rest of the series, Patches 1-2 had
kernel support. Patches 3-4 added
libbpf support. Patch 5 added bpftool
support. Patches 6-10 added various selftests.
Patch 11 added documentation for the new kind.

  [1] https://reviews.llvm.org/D106614
  [2] https://reviews.llvm.org/D106621
  [3] https://reviews.llvm.org/D106622
  [4] https://reviews.llvm.org/D109560

Changelog:
  v2 -> v3:
    - put NR_BTF_KINDS and BTF_KIND_MAX into enum as well
    - check component_idx earlier (check_meta stage) in kernel
    - add more tests
    - fix misc nits
  v1 -> v2:
    - BTF ELF format changed in llvm ([4] above),
      so cross-board change to use the new format.
    - Clarified in commit message that BTF_KIND_TAG
      is not emitted by bpftool btf dump format c.
    - Fix various comments from Andrii.
====================

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>