tools: ynl: c: correct reverse decode of empty attrs

tools: ynl: c: correct reverse decode of empty attrs

netlink reports which attribute was incorrect by sending back
an attribute offset. Offset points to the address of struct nlattr,
but to interpret the type we also need the nesting path.
Attribute IDs have different meaning in different nests
of the same message.

Correct the condition for "is the offset within current attribute".
ynl_attr_data_len() does not include the attribute header,
so the end offset was off by 4 bytes.

This means that we'd always skip over flags and empty nests.

The devmem tests, for example, issues an invalid request with
empty queue nests, resulting in the following error:

  YNL failed: Kernel error: missing attribute: .queues.ifindex

The message is incorrect, "queues" nest does not have an "ifindex"
attribute defined. With this fix we decend correctly into the nest:

  YNL failed: Kernel error: missing attribute: .queues.id

Fixes: 86878f14d71a ("tools: ynl: user space helpers")
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Link: https://patch.msgid.link/20250124012130.1121227-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>