Track which nests are recursive. Non-recursive nesting gets
rendered in C as directly nested structs. For recursive
ones we need to put a pointer in, rather than full struct.

Track this information, no change to generated code, yet.
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://lore.kernel.org/r/20231213231432.2944749-6-kuba@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>