ethtool: rss: echo the context number back

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: 7112a04 ("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>

Author: kuba-moo

Documentation/netlink/specs/ethtool.yaml

@@ -1757,6 +1757,7 @@ operations:
         reply:
           attributes:
             - header
+            - context
             - hfunc
             - indir
             - hkey

Documentation/networking/ethtool-netlink.rst

@@ -1875,6 +1875,7 @@ Kernel response contents:
 
 =====================================  ======  ==========================
   ``ETHTOOL_A_RSS_HEADER``             nested  reply header
+  ``ETHTOOL_A_RSS_CONTEXT``            u32     context number
   ``ETHTOOL_A_RSS_HFUNC``              u32     RSS hash func
   ``ETHTOOL_A_RSS_INDIR``              binary  Indir table bytes
   ``ETHTOOL_A_RSS_HKEY``               binary  Hash key bytes

net/ethtool/rss.c

@@ -111,7 +111,8 @@ rss_reply_size(const struct ethnl_req_info *req_base,
 	const struct rss_reply_data *data = RSS_REPDATA(reply_base);
 	int len;
 
-	len = nla_total_size(sizeof(u32)) +	/* _RSS_HFUNC */
+	len = nla_total_size(sizeof(u32)) +	/* _RSS_CONTEXT */
+	      nla_total_size(sizeof(u32)) +	/* _RSS_HFUNC */
 	      nla_total_size(sizeof(u32)) +	/* _RSS_INPUT_XFRM */
 	      nla_total_size(sizeof(u32) * data->indir_size) + /* _RSS_INDIR */
 	      nla_total_size(data->hkey_size);	/* _RSS_HKEY */
@@ -124,6 +125,11 @@ rss_fill_reply(struct sk_buff *skb, const struct ethnl_req_info *req_base,
 	       const struct ethnl_reply_data *reply_base)
 {
 	const struct rss_reply_data *data = RSS_REPDATA(reply_base);
+	struct rss_req_info *request = RSS_REQINFO(req_base);
+
+	if (request->rss_context &&
+	    nla_put_u32(skb, ETHTOOL_A_RSS_CONTEXT, request->rss_context))
+		return -EMSGSIZE;
 
 	if ((data->hfunc &&
 	     nla_put_u32(skb, ETHTOOL_A_RSS_HFUNC, data->hfunc)) ||