bonding: correctly verify for the first slave in bond_enslave

After commit 1f718f0 ("bonding: populate
neighbour's private on enslave"), we've moved the actual 'linking' in the
end of the function - so that, once linked, the slave is ready to be used,
and is not still in the process of enslaving.

However, 802.3ad verified if it's the first slave by looking at the

if (bond_first_slave(bond) == new_slave)

which, because we've moved the linking to the end, became broken - on the
first slave bond_first_slave(bond) returns NULL.

Fix this by verifying if the prev_slave, that equals bond_last_slave(), is
actually populated - if it is - then it's not the first slave, and vice
versa.

CC: Jay Vosburgh <fubar@us.ibm.com>
CC: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/bonding/bond_main.c

@@ -1557,7 +1557,7 @@ int bond_enslave(struct net_device *bond_dev, struct net_device *slave_dev)
 		 */
 		bond_set_slave_inactive_flags(new_slave);
 		/* if this is the first slave */
-		if (bond_first_slave(bond) == new_slave) {
+		if (!prev_slave) {
 			SLAVE_AD_INFO(new_slave).id = 1;
 			/* Initialize AD with the number of times that the AD timer is called in 1 second
 			 * can be called only after the mac address of the bond is set