net/core: Add VF link state control

Add netlink directives and ndo entry to allow for controling
VF link, which can be in one of three states:

Auto - VF link state reflects the PF link state (default)

Up - VF link state is up, traffic from VF to VF works even if
the actual PF link is down

Down - VF link state is down, no traffic from/to this VF, can be of
use while configuring the VF

Signed-off-by: Rony Efraim <ronye@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>