[IPV6]: Do mandatory IPv6 tunnel endpoint checks in realtime

Doing the mandatory tunnel endpoint checks when the tunnel is set up
isn't enough as interfaces can go up or down and addresses can be
added or deleted after this. The checks need to be done realtime when
the tunnel is processing a packet.

Signed-off-by: Ville Nuorvala <vnuorval@tcs.hut.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>