selftests/bpf: Move newer bpf_iter_* type redefining to a new header file

Commit b9f4c01f3e0b ("selftest/bpf: Make bpf_iter selftest
compilable against old vmlinux.h") and Commit dda18a5c0b75
("selftests/bpf: Convert bpf_iter_test_kern{3, 4}.c to define
own bpf_iter_meta") redefined newly introduced types
in bpf programs so the bpf program can still compile
properly with old kernels although loading may fail.

Since this patch set introduced new types and the same
workaround is needed, so let us move the workaround
to a separate header file so they do not clutter
bpf programs.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20200623230816.3988656-1-yhs@fb.com

selftests/bpf: Convert bpf_iter_test_kern{3, 4}.c to define own bpf_iter_meta

b9f4c01f3e0b ("selftest/bpf: Make bpf_iter selftest compilable against old vmlinux.h")
missed the fact that bpf_iter_test_kern{3,4}.c are not just including
bpf_iter_test_kern_common.h and need similar bpf_iter_meta re-definition
explicitly.

Fixes: b9f4c01f3e0b ("selftest/bpf: Make bpf_iter selftest compilable against old vmlinux.h")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200519192341.134360-1-andriin@fb.com

tools/bpf: selftests: Add bpf_iter selftests

The added test includes the following subtests:
- test verifier change for btf_id_or_null
- test load/create_iter/read for
ipv6_route/netlink/bpf_map/task/task_file
- test anon bpf iterator
- test anon bpf iterator reading one char at a time
- test file bpf iterator
- test overflow (single bpf program output not overflow)
- test overflow (single bpf program output overflows)
- test bpf prog returning 1

The ipv6_route tests the following verifier change
- access fields in the variable length array of the structure.

The netlink load tests the following verifier change
- put a btf_id ptr value in a stack and accessible to
tracing/iter programs.

The anon bpf iterator also tests link auto attach through skeleton.

$ test_progs -n 2
#2/1 btf_id_or_null:OK
#2/2 ipv6_route:OK
#2/3 netlink:OK
#2/4 bpf_map:OK
#2/5 task:OK
#2/6 task_file:OK
#2/7 anon:OK
#2/8 anon-read-one-char:OK
#2/9 file:OK
#2/10 overflow:OK
#2/11 overflow-e2big:OK
#2/12 prog-ret-1:OK
#2 bpf_iter:OK
Summary: 1/12 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200509175923.2477637-1-yhs@fb.com