vdpa: Enable user to set mtu of the vdpa device

vdpa: Enable user to set mtu of the vdpa device

Implement mtu setting for vdpa device.

$ vdpa mgmtdev show
vdpasim_net:
  supported_classes net

Add the device with mac address and mtu:
$ vdpa dev add name bar mgmtdev vdpasim_net mac 00:11:22:33:44:55 mtu 9000

In above command only mac address or only mtu can also be set.

View the config after setting:
$ vdpa dev config show
bar: mac 00:11:22:33:44:55 link up link_announce false mtu 9000

Signed-off-by: Parav Pandit <parav@nvidia.com>
Signed-off-by: David Ahern <dsahern@kernel.org>