scripts/bpf_doc.py: update logic to not assume sequential enum values

scripts/bpf_doc.py: update logic to not assume sequential enum values

Relax bpf_doc.py's expectation of all BPF_FUNC_xxx enumerators having
sequential values increasing by one. Instead, only make sure that
relative order of BPF helper descriptions in comments matches
enumerators definitions order.

Also additionally make sure that helper IDs are not duplicated.

And also make sure that for cases when we have multiple descriptions for
the same BPF helper (e.g., for bpf_get_socket_cookie()), all such
descriptions are grouped together.

Such checks should capture all the same (and more) issues in upstream
UAPI headers, but also handle backported kernels correctly.

Reported-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/r/20221006042452.2089843-2-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>