mlxsw: spectrum_router: Handle MTU change of GRE netdevs

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: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>