ipv6: Regenerate host route according to node pointer upon loopback up

When the loopback device is brought back up we need to check if the host
route attached to the address is still in the FIB and regenerate one in
case it's not.

Host routes using the loopback device are always inserted into and
removed from the FIB under RTNL (under which this function is called),
so we can test their node pointer instead of the reference count in
order to check if the route is in the FIB or not.

Tested using the following script from Nicolas mentioned in
commit a220445f ("ipv6: correctly add local routes when lo goes up"):

$ ip link add dummy1 type dummy
$ ip link set dummy1 up
$ ip link set lo down ; ip link set lo up

The host route is correctly regenerated.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>