[IPIP]: Allow rebinding the tunnel to another interface

Once created, an IP tunnel can't be bound to another device.
(reported as https://bugzilla.redhat.com/show_bug.cgi?id=419671

)

To reproduce:

# create a tunnel:
ip tunnel add tunneltest0 mode ipip remote 10.0.0.1 dev eth0
# try to change the bounding device from eth0 to eth1:
ip tunnel change tunneltest0 dev eth1
# show the result:
ip tunnel show tunneltest0

tunneltest0: ip/ip  remote 10.0.0.1  local any  dev eth0  ttl inherit

Notice the bound device has not changed from eth0 to eth1.

This patch fixes it. When changing the binding, it also recalculates the
MTU according to the new bound device's MTU.

If the change is acceptable, I'll do the same for GRE and SIT tunnels.

Signed-off-by: Michal Schmidt <mschmidt@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>