net: respect MTU set via ioctl

Fixes #372.

ioctl handler for virtio-net was ignoring MTU changes. On GCE the
interface MTU is less than the default (1460 vs 1500). Not respecting
MTU led to communication failures when large packets were sent, for
example:

IP truncated-ip - OSV.8443 > TOMEK.52012: Flags [.], seq 1:1449, ack 179, win 1040, options [nop,nop,TS val 476582182 ecr 62762835], length 1448
IP 10.240.0.1 > OSV: ICMP TOMEK unreachable - need to frag (mtu 1460), length 36

The same problem existed for vmxnet3 driver.

The fix is to remove handler for SIOCSIFMTU, which does nothing but
logging, so that we fall through to ether_ioctl() in the default
clause.

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>