tools: ynl-gen: support passing selector to a nest

In rtnetlink all submessages had the selector at the same level
of nesting as the submessage. We could refer to the relevant
attribute from the current struct. In TC, stats are one level
of nesting deeper than "kind". Teach the code-gen about structs
which need to be passed a selector by the caller for parsing.

Because structs are "topologically sorted" one pass of propagating
the selectors down is enough.

For generating netlink message we depend on the presence bits
so no selector passing needed there.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250520161916.413298-7-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>