MFC r319491:

Skip setting the MTU in the netfront driver (xn# devices) if the new MTU
is the same as the old MTU. In particular, on Amazon EC2 "T2" instances
without this change, the network interface is reinitialized every 30
minutes due to the MTU being (re)set when a new DHCP lease is obtained.

Approved by: re (delphij)