Merge branch 'link_query-bpf_iter'

Merge branch 'link_query-bpf_iter'

Yonghong Song says:

====================
"link" has been an important concept for bpf ecosystem to connect
bpf program with other properties. Currently, the information related
information can be queried from userspace through bpf command
BPF_LINK_GET_NEXT_ID, BPF_LINK_GET_FD_BY_ID and BPF_OBJ_GET_INFO_BY_FD.
The information is also available by "cating" /proc/<pid>/fdinfo/<link_fd>.
Raw_tracepoint, tracing, cgroup, netns and xdp links are already
supported in the kernel and bpftool.

This patch added support for bpf iterator. Patch #1 added generic support
for link querying interface. Patch #2 implemented callback functions
for map element bpf iterators. Patch #3 added bpftool support.

Changelogs:
  v3 -> v4:
    . return target specific link_info even if target_name buffer
      is empty. (Andrii)
  v2 -> v3:
    . remove extra '\t' when fdinfo prints map_id to make parsing
      consistent. (Andrii)
  v1 -> v2:
    . fix checkpatch.pl warnings. (Jakub)
====================

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