net: ethtool: Fix suspicious rcu_dereference usage

The __ethtool_get_ts_info function can be called with or without the
rtnl lock held. When the rtnl lock is not held, using rtnl_dereference()
triggers a warning due to the lack of lock context.

Add an rcu_read_lock() to ensure the lock is acquired and to maintain
synchronization.

Reported-by:  <syzbot+a344326c05c98ba19682@syzkaller.appspotmail.com>
Closes: https://lore.kernel.org/netdev/676147f8.050a0220.37aaf.0154.GAE@google.com/


Fixes: b9e3f7dc ("net: ethtool: tsinfo: Enhance tsinfo to support several hwtstamp by net topology")
Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20241220083741.175329-1-kory.maincent@bootlin.com


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