Merge branch 'bpf-uapi-enums'

Andrii Nakryiko says:

====================
Convert BPF-related UAPI constants, currently defined as #define macro, into
anonymous enums. This has no difference in terms of usage of such constants in
C code (they are still could be used in all the compile-time contexts that
`#define`s can), but they are recorded as part of DWARF type info, and
subsequently get recorded as part of kernel's BTF type info. This allows those
constants to be emitted as part of vmlinux.h auto-generated header file and be
used from BPF programs. Which is especially convenient for all kinds of BPF
helper flags and makes CO-RE BPF programs nicer to write.

libbpf's btf_dump logic currently assumes enum values are signed 32-bit
values, but that doesn't match a typical case, so switch it to emit unsigned
values. Once BTF encoding of BTF_KIND_ENUM is extended to capture signedness
properly, this will be made more flexible.

As an immediate validation of the approach, runqslower's copy of
BPF_F_CURRENT_CPU #define is dropped in favor of its enum variant from
vmlinux.h.

v2->v3:
- convert only constants usable from BPF programs (BPF helper flags, map
  create flags, etc) (Alexei);
v1->v2:
- fix up btf_dump test to use max 32-bit unsigned value instead of negative one.
====================

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