netdev-linux: Cache error code from mtu ioctl.

netdev-linux: Cache error code from mtu ioctl.

netdev linux devices uses mtu ioctl to get and set MTU for a device.
By caching error code from ioctl we can reduce number of ioctl calls
for device which is unregistered from system.
netdev notification is used to update mtu which saves get-mtu-ioctl.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>