plip: avoid rcu debug splat

WARNING: suspicious RCU usage
5.2.0-rc2-00605-g2638eb8b50cfc #1 Not tainted
drivers/net/plip/plip.c:1110 suspicious rcu_dereference_check() usage!

plip_open is called with RTNL held, switch to the correct helper.

Fixes: 2638eb8b

 ("net: ipv4: provide __rcu annotation for ifa_list")
Reported-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20220807115304.13257-1-fw@strlen.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>