tools: ynl: correctly handle overrides of fields in subset

tools: ynl: correctly handle overrides of fields in subset

We stated in documentation [1] and previous discussions [2]
that the need for overriding fields in members of subsets
is anticipated. Implement it.

Since each attr is now a new object we need to make sure
that the modifications are propagated. Specifically C codegen
wants to annotate which attrs are used in requests and replies
to generate the right validation artifacts.

[1] https://docs.kernel.org/next/userspace-api/netlink/specs.html#subset-of
[2] https://lore.kernel.org/netdev/20231004171350.1f59cd1d@kernel.org/

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250107022820.2087101-2-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>