netdev: Allow get_mtu and set_mtu provider functions to be null.

netdev: Allow get_mtu and set_mtu provider functions to be null.

Most netdev provider functions are allowed to be null if the implementation
does not support this feature.  This commit adds this feature for get_mtu
and set_mtu, and changes netdev-vport to take advantage of it.

Also, changes netdev_get_mtu() to report an MTU of 0 on error, instead of
leaving the MTU indeterminate.