ethtool: rss: echo the context number back

[ Upstream commit f96aae91 ]

The response to a GET request in Netlink should fully identify
the queried object. RSS_GET accepts context id as an input,
so it must echo that attribute back to the response.

After (assuming context 1 has been created):

  $ ./cli.py --spec netlink/specs/ethtool.yaml \
             --do rss-get \
	     --json '{"header": {"dev-index": 2}, "context": 1}'
  {'context': 1,
   'header': {'dev-index': 2, 'dev-name': 'eth0'},
  [...]

Fixes: 7112a046

 ("ethtool: add netlink based get rss support")
Acked-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Joe Damato <jdamato@fastly.com>
Link: https://patch.msgid.link/20240724234249.2621109-3-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>