Add IFLA_VF_TRUST message to trust the VF.
PF can accept some privileged operation from the trusted VF.
For example, ixgbe PF doesn't allow to enable VF promiscuous mode until
the VF is trusted because it may hurt performance.

To trust VF.
 # ip link set dev eth0 vf 1 trust on

To untrust VF.
 # ip link set dev eth0 vf 1 trust off
Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>