net/virtio: set MTU

net/virtio: set MTU

Virtio interfaces do not currently allow the user to specify a particular
Maximum Transmission Unit (MTU). Consequently, the MTU of Virtio interfaces
is typically set to the Ethernet default value of 1500.
This is problematic in the case of cloud deployments, in which a specific
(and potentially non-standard) MTU needs to be set by a DHCP server, which
needs to be honored by all interfaces across the traffic path.To acheive
this Virtio interfaces should support setting of MTU.
In case when GRE/VXLAN tunneling is used for internal communication, there
will be an overhead added by the infrastructure in the packet over and
above the ETHER MTU of 1518. So to take care of this overhead in these
cases the DHCP server corrects the L3 MTU to 1454. But since virtio
interfaces was not having the MTU set functionality that MTU sent by the
DHCP server was ignored and the instance will still send packets with 1500
MTU which after encapsulation will become more than 1518 and eventually
gets dropped in the infrastructure.
By adding an additional 'set_mtu' function to the Virtio driver, we can
honor the MTU sent by the DHCP server. The dhcp server/controller can
then leverage this 'set_mtu' functionality to resolve the above
mentioned issue of packets getting dropped due to incorrect size.

Signed-off-by: Souvik Dey <sodey@sonusnet.com>
Reviewed-by: Mark Kavanagh <mark.b.kavanagh@intel.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>