virtio: don't expose u16 in userspace api

Programs using virtio headers outside of kernel will no longer
build because u16 type does not exist in userspace. All user ABI
must use __u16 typedef instead.

Bug introduce by:
  commit 986a4f4d
  Author: Jason Wang <jasowang@redhat.com>
  Date:   Fri Dec 7 07:04:56 2012 +0000

    virtio_net: multiqueue support

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>