Merge branch 'btf_get_next_id'

Merge branch 'btf_get_next_id'

Quentin Monnet says:

====================
This set adds a new command BPF_BTF_GET_NEXT_ID to the bpf() system call,
adds the relevant API function in libbpf, and uses it in bpftool to list
all BTF objects loaded on the system (and to dump the ids of maps and
programs associated with them, if any).

The main motivation of listing BTF objects is introspection and debugging
purposes. By getting BPF program and map information, it should already be
possible to list all BTF objects associated to at least one map or one
program. But there may be unattached BTF objects, held by a file descriptor
from a user space process only, and we may want to list them too.

As a side note, it also turned useful for examining the BTF objects
attached to offloaded programs, which would not show in program information
because the BTF id is not copied when retrieving such info. A fix is in
progress on that side.

v2:
- Rebase patch with new libbpf function on top of Andrii's changes
  regarding libbpf versioning.
====================

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