bonding: check slave set command firstly

This patch is a little improvement. If user use the
command shown as below, we should print the info [1]
instead of [2]. The eth0 exists actually, and it may
confuse user.

$ echo "eth0" > /sys/class/net/bond4/bonding/slaves

[1] "bond4: no command found in slaves file - use +ifname or -ifname"
[2] "write error: No such device"
Signed-off-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/bonding/bond_options.c

@@ -1375,6 +1375,7 @@ static int bond_option_slaves_set(struct bonding *bond,
 	sscanf(newval->string, "%16s", command); /* IFNAMSIZ*/
 	ifname = command + 1;
 	if ((strlen(command) <= 1) ||
+	    (command[0] != '+' && command[0] != '-') ||
 	    !dev_valid_name(ifname))
 		goto err_no_cmd;
 
@@ -1398,6 +1399,7 @@ static int bond_option_slaves_set(struct bonding *bond,
 		break;
 
 	default:
+		/* should not run here. */
 		goto err_no_cmd;
 	}