net: ethtool: move rxfh_fields callbacks under the rss_lock

Netlink code will want to perform the RSS_SET operation atomically
under the rss_lock. sfc wants to hold the rss_lock in rxfh_fields_get,
which makes that difficult. Lets move the locking up to the core
so that for all driver-facing callbacks rss_lock is taken consistently
by the core.

Link: https://patch.msgid.link/20250626202848.104457-3-kuba@kernel.org


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