net: vrf: Switch dst dev to loopback on device delete

Attempting to delete a VRF device with a socket bound to it can stall:

  unregister_netdevice: waiting for red to become free. Usage count = 1

The unregister is waiting for the dst to be released and with it
references to the vrf device. Similar to dst_ifdown switch the dst
dev to loopback on delete for all of the dst's for the vrf device
and release the references to the vrf device.

Fixes: 193125db ("net: Introduce VRF device driver")
Fixes: 35402e31 ("net: Add IPv6 support to VRF device")
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>