ethtool: rss: fix hiding unsupported fields in dumps

Commit ec6e57b ("ethtool: rss: don't report key if device
doesn't support it") intended to stop reporting key fields for
additional rss contexts if device has a global hashing key.

Later we added dump support and the filtering wasn't properly
added there. So we end up reporting the key fields in dumps
but not in dos:

  # ./pyynl/cli.py --spec netlink/specs/ethtool.yaml --do rss-get \
		--json '{"header": {"dev-index":2}, "context": 1 }'
  {
     "header": { ... },
     "context": 1,
     "indir": [0, 1, 2, 3, ...]]
  }

  # ./pyynl/cli.py --spec netlink/specs/ethtool.yaml --dump rss-get
  [
     ... snip context 0 ...
     { "header": { ... },
       "context": 1,
       "indir": [0, 1, 2, 3, ...],
 ->    "input_xfrm": 255,
 ->    "hfunc": 1,
 ->    "hkey": "000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000"
     }
  ]

Hide these fields correctly.

The drivers/net/hw/rss_ctx.py selftest catches this when run on
a device with single key, already:

  # Check| At /root/./ksft-net-drv/drivers/net/hw/rss_ctx.py, line 381, in test_rss_context_dump:
  # Check|     ksft_ne(set(data.get('hkey', [1])), {0}, "key is all zero")
  # Check failed {0} == {0} key is all zero
  not ok 8 rss_ctx.test_rss_context_dump

Fixes: f612290 ("ethtool: rss: support dumping RSS contexts")
Reviewed-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Joe Damato <jdamato@fastly.com>
Link: https://patch.msgid.link/20250201013040.725123-2-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ethtool/rss.c

@@ -107,6 +107,8 @@ rss_prepare_ctx(const struct rss_req_info *request, struct net_device *dev,
 	u32 total_size, indir_bytes;
 	u8 *rss_config;
 
+	data->no_key_fields = !dev->ethtool_ops->rxfh_per_ctx_key;
+
 	ctx = xa_load(&dev->ethtool->rss_ctx, request->rss_context);
 	if (!ctx)
 		return -ENOENT;
@@ -153,7 +155,6 @@ rss_prepare_data(const struct ethnl_req_info *req_base,
 		if (!ops->cap_rss_ctx_supported && !ops->create_rxfh_context)
 			return -EOPNOTSUPP;
 
-		data->no_key_fields = !ops->rxfh_per_ctx_key;
 		return rss_prepare_ctx(request, dev, data, info);
 	}