net: ipv6: Release route when device is unregistering

Roopa reported attempts to delete a bond device that is referenced in a
multipath route is hanging:

$ ifdown bond2    # ifupdown2 command that deletes virtual devices
unregister_netdevice: waiting for bond2 to become free. Usage count = 2

Steps to reproduce:
    echo 1 > /proc/sys/net/ipv6/conf/all/ignore_routes_with_linkdown
    ip link add dev bond12 type bond
    ip link add dev bond13 type bond
    ip addr add 2001:db8:2::0/64 dev bond12
    ip addr add 2001:db8:3::0/64 dev bond13
    ip route add 2001:db8:33::0/64 nexthop via 2001:db8:2::2 nexthop via 2001:db8:3::2
    ip link del dev bond12
    ip link del dev bond13

The root cause is the recent change to keep routes on a linkdown. Update
the check to detect when the device is unregistering and release the
route for that case.

Fixes: a1a22c12 ("net: ipv6: Keep nexthop of multipath route on admin down")
Reported-by: NRoopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: NDavid Ahern <dsahern@gmail.com>
Acked-by: NRoopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -4262,6 +4262,11 @@ static inline const char *netdev_name(const struct net_device *dev)
 	return dev->name;
 }
 
+static inline bool netdev_unregistering(const struct net_device *dev)
+{
+	return dev->reg_state == NETREG_UNREGISTERING;
+}
+
 static inline const char *netdev_reg_state(const struct net_device *dev)
 {
 	switch (dev->reg_state) {

net/ipv6/route.c

@@ -2804,6 +2804,7 @@ static int fib6_ifdown(struct rt6_info *rt, void *arg)
 	if ((rt->dst.dev == dev || !dev) &&
 	    rt != adn->net->ipv6.ip6_null_entry &&
 	    (rt->rt6i_nsiblings == 0 ||
+	     (dev && netdev_unregistering(dev)) ||
 	     !rt->rt6i_idev->cnf.ignore_routes_with_linkdown))
 		return -1;