ethtool: rss: avoid modifying the RSS context response

ethtool: rss: avoid modifying the RSS context response

Gemini says that we're modifying the RSS_CREATE response skb.
I think it's right, the comment says that unicast() should
unshare the skb but I'm not entirely sure what I meant there.
netlink_trim() does a copy but only if skb is not well sized
(it's at least 2x larger than necessary for the payload).

Fixes: a166ab7816c5 ("ethtool: rss: support creating contexts via Netlink")
Link: https://patch.msgid.link/20260522230647.1705600-2-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>