net: remove MTU limits for dummy and ifb device

net: remove MTU limits for dummy and ifb device

These two drivers (dummy and ifb) call ether_setup(), after commit
61e84623ace3 ("net: centralize net_device min/max MTU checking"), the
range of mtu is [min_mtu, max_mtu], which is [68, 1500] by default.

These two devices should not have limits on MTU. This patch set their
min_mtu/max_mtu to 0. So that dev_set_mtu() will not check the mtu range,
and can be set with any value.

CC: Eric Dumazet <[email protected]>
CC: Sabrina Dubroca <[email protected]>
Signed-off-by: Zhang Shengju <[email protected]>
Signed-off-by: David S. Miller <[email protected]>