selftests, bpf: Make docs tests fail more reliably

Previously, if rst2man caught errors, then these would be ignored and
the output file would be written anyway. This would allow developers to
introduce regressions in the docs comments in the BPF headers.

Additionally, even if you instruct rst2man to fail out, it will still
write out to the destination target file, so if you ran the tests twice
in a row it would always pass. Use a temporary file for the initial run
to ensure that if rst2man fails out under "--strict" mode, subsequent
runs will not automatically pass.

Tested via ./tools/testing/selftests/bpf/test_doc_build.sh

Signed-off-by: Joe Stringer <joe@cilium.io>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20210608015756.340385-1-joe@cilium.io

selftests/bpf: Test syscall command parsing

Add building of the bpf(2) syscall commands documentation as part of the
docs building step in the build. This allows us to pick up on potential
parse errors from the docs generator script as part of selftests.

The generated manual pages here are not intended for distribution, they
are just a fragment that can be integrated into the other static text of
bpf(2) to form the full manual page.

Signed-off-by: Joe Stringer <joe@cilium.io>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210302171947.2268128-14-joe@cilium.io

selftests/bpf: Templatize man page generation

Previously, the Makefile here was only targeting a single manual page so
it just hardcoded a bunch of individual rules to specifically handle
build, clean, install, uninstall for that particular page.

Upcoming commits will generate manual pages for an additional section,
so this commit prepares the makefile first by converting the existing
targets into an evaluated set of targets based on the manual page name
and section.

Signed-off-by: Joe Stringer <joe@cilium.io>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/bpf/20210302171947.2268128-13-joe@cilium.io

tools/bpf: Remove bpf-helpers from bpftool docs

This logic is used for validating the manual pages from selftests, so
move the infra under tools/testing/selftests/bpf/ and rely on selftests
for validation rather than tying it into the bpftool build.

Signed-off-by: Joe Stringer <joe@cilium.io>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/bpf/20210302171947.2268128-12-joe@cilium.io