tools/net/ynl: fix sub-message key lookup for nested attributes

Use the correct attribute space for sub-message key lookup in nested
attributes when adding attributes. This fixes rt_link where the "kind"
key and "data" sub-message are nested attributes in "linkinfo".

For example:

./tools/net/ynl/cli.py \
    --create \
    --spec Documentation/netlink/specs/rt_link.yaml \
    --do newlink \
    --json '{"link": 99,
             "linkinfo": { "kind": "vlan", "data": {"id": 4 } }
             }'

Signed-off-by: Donald Hunter <donald.hunter@gmail.com>
Fixes: ab463c43 ("tools/net/ynl: Add support for encoding sub-messages")
Link: https://patch.msgid.link/20241213130711.40267-1-donald.hunter@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>