net: ipv6/l3mdev: Move host route on saved address if necessary

Commit f1705ec1

 allows IPv6 addresses to be retained on a link down.
The address can have a cached host route which can point to the wrong
FIB table if the L3 enslavement is changed (e.g., route can point to local
table instead of VRF table if device is added to an L3 domain).

On link up check the table of the cached host route against the FIB
table associated with the device and correct if needed.

Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>