bonding: add sysfs /slave dir for bond slave devices.

Add sub-directory under /sys/class/net/<interface>/slave with
read-only attributes for slave.  Directory only appears when
<interface> is a slave.

$ tree /sys/class/net/eth2/slave/
/sys/class/net/eth2/slave/
├── ad_aggregator_id
├── link_failure_count
├── mii_status
├── perm_hwaddr
├── queue_id
└── state

$ cat /sys/class/net/eth2/slave/*
2
0
up
40:02:10:ef:06:01
0
active

Signed-off-by: Scott Feldman <sfeldma@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/bonding/bond_procfs.c

@@ -159,18 +159,6 @@ static void bond_info_show_master(struct seq_file *seq)
 	}
 }
 
-static const char *bond_slave_link_status(s8 link)
-{
-	static const char * const status[] = {
-		[BOND_LINK_UP] = "up",
-		[BOND_LINK_FAIL] = "going down",
-		[BOND_LINK_DOWN] = "down",
-		[BOND_LINK_BACK] = "going back",
-	};
-
-	return status[link];
-}
-
 static void bond_info_show_slave(struct seq_file *seq,
 				 const struct slave *slave)
 {