Update MTU of overlay loopback in accordance with the setting on the
tunnel netdevice.

Fixes: 0063587d ("mlxsw: spectrum: Support decap-only IP-in-IP tunnels")
Signed-off-by: NPetr Machata <petrm@mellanox.com>
Signed-off-by: NIdo Schimmel <idosch@mellanox.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>