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,
causing packets to be dropped, along with annoying syslog messages about
the link state changing.

As a side note, the behaviour this commit fixes was responsible for
exposing the locking problems fixed via r318523 and r318631.

Maintainers of other network interface drivers may wish to consider making
the corresponding change; the handling of SIOCSIFMTU does not seem to
exhibit a great deal of consistency between drivers.

MFC after: 1 week