tools: ynl-gen: fix collecting global policy attrs

We look for attributes inside do.request, but there's another
layer of nesting in the spec, look inside do.request.attributes.

This bug had no effect as all global policies we generate (fou)
seem to be full, anyway, and we treat full and empty the same.

Next patch will change the treatment of empty policies.

Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20230824003056.1436637-4-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>