bonding: allow carrier and link status to determine link state

In a mixed environment it may be difficult to tell if your hardware
support carrier, if it does not it can always report true. With a new
use_carrier option of 2, we can check both carrier and link status
sequentially, instead of one or the other

Signed-off-by: Debabrata Banerjee <dbanerje@akamai.com>
Signed-off-by: David S. Miller <davem@davemloft.net>