bonding: show all the link status of slaves

There are four link statuses of a bonding slave, the procfs
code shows a wrong status when using downdelay/updelay:

	(slave->link == BOND_LINK_UP) ?  "up" : "down"

It doesn't respect the rest two statuses. This patch fixes it.

Cc: Jay Vosburgh <fubar@us.ibm.com>
Cc: Andy Gospodarek <andy@greyhouse.net>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>