net/ipv6: fix LOCKDEP issue in rt6_remove_exception_rt()

rt6_remove_exception_rt() is called under rcu_read_lock() only.

We lock rt6_exception_lock a bit later, so we do not hold
rt6_exception_lock yet.

Fixes: 8a14e46f ("net/ipv6: Fix missing rcu dereferences on from")
Signed-off-by: NEric Dumazet <edumazet@google.com>
Reported-by: Nsyzbot <syzkaller@googlegroups.com>
Cc: David Ahern <dsahern@gmail.com>
Acked-by: NDavid Ahern <dsahern@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/route.c

@@ -1546,8 +1546,7 @@ static int rt6_remove_exception_rt(struct rt6_info *rt)
 	struct fib6_info *from;
 	int err;
 
-	from = rcu_dereference_protected(rt->from,
-					 lockdep_is_held(&rt6_exception_lock));
+	from = rcu_dereference(rt->from);
 	if (!from ||
 	    !(rt->rt6i_flags & RTF_CACHE))
 		return -EINVAL;