net: ethtool: take rss_lock for all rxfh changes

Always take the rss_lock in ethtool_set_rxfh(). We will want to
make a similar change in ethtool_set_rxfh_fields() and some
drivers lock that callback regardless of rss context ID being set.
Having some callbacks locked unconditionally and some only if
context ID is set would be very confusing.

ethtool handling is under rtnl_lock, so rss_lock is very unlikely
to ever be congested.

Link: https://patch.msgid.link/20250626202848.104457-2-kuba@kernel.org


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