ethtool: rss: make sure dump takes the rss lock

After commit 040cef30 ("net: ethtool: move get_rxfh callback
under the rss_lock") we're expected to take rss_lock around get.
Switch dump to using the new prep helper and move the locking into it.

Link: https://patch.msgid.link/20250708220640.2738464-2-kuba@kernel.org


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