tools: ynl-gen: make sure we validate subtype of array-nest

tools: ynl-gen: make sure we validate subtype of array-nest

ArrayNest AKA indexed-array support currently skips inner type
validation. We count the attributes and then we parse them,
make sure we call validate, too. Otherwise buggy / unexpected
kernel response may lead to crashes.

Fixes: be5bea1cc0bf ("net: add basic C code generators for Netlink")
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20250414211851.602096-5-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>