net: ethtool: move get_rxfh callback under the rss_lock

We already call get_rxfh under the rss_lock when we read back
context state after changes. Let's be consistent and always
hold the lock. The existing callers are all under rtnl_lock
so this should make no difference in practice, but it makes
the locking rules far less confusing IMHO. Any RSS callback
and any access to the RSS XArray should hold the lock.

Link: https://patch.msgid.link/20250626202848.104457-4-kuba@kernel.org


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