datapath: Strip down vport interface : OVS_VPORT_ATTR_MTU

datapath: Strip down vport interface : OVS_VPORT_ATTR_MTU

There is no need to have vport attribute MTU (OVS_VPORT_ATTR_MTU) as
linux net-dev-ioctl can be used to get/set MTU for linux device.
Following patch removes OVS_VPORT_ATTR_MTU from datapath protocol.

This patch also adds netdev_set_mtu interface. So that MTU adjustments
can be done from OVS userspace. get_mtu() interface is also changed, now
get_mtu() returns EOPNOTSUPP rather than returning 0 and setting *pmtu
to INT_MAX in case there is no MTU attribute for given device.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>