Set MTU on bond master as opposed to a bond slave

Set MTU on bond master as opposed to a bond slave

Prior to this commit, the MTU setting was applied to a bond slave
interface.  In older versions of the Linux kernel, this setting
propagated to the bond master and the other bond slaves associated with
the master.  In recent versions of the kernel (observed in Linux 4.12),
increasing the MTU of a slave does not automatically increase the MTU of
the master.  This allows for more flexibility but requires the MTU of
the master to be changed manually.

Ideally, the MTU setting should be applied to the bond master and the
setting will propagate to the bond slaves, since the slaves are required
to have a MTU that is greater than or equal to the bond master.