Merge branch 'btf-uapi-cleanups'

Martin KaFai Lau says:

====================
This patch set makes some changes to cleanup the unused
bits in BTF uapi.  It also makes the btf_header extensible.

Please see individual patches for details.

v2:
- Remove NR_SECS from patch 2
- Remove "unsigned" check on array->index_type from patch 3
- Remove BTF_INT_VARARGS and further limit BTF_INT_ENCODING
  from 8 bits to 4 bits in patch 4
- Adjustments in test_btf.c to reflect changes in v2
====================

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>