[IPV6] sit: Rebinding of SIT tunnels to other interfaces

This is similar to the change already done for IPIP tunnels.

Once created, a SIT tunnel can't be bound to another device.
To reproduce:

# create a tunnel:
ip tunnel add tunneltest0 mode sit 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: ipv6/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.

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