bonding: fix panic when taking bond interface down before removing module

A panic was discovered with bonding when using mode 5 or 6 and trying to
remove the slaves from the bond after the interface was taken down.
When calling 'ifconfig bond0 down' the following happens:

    bond_close()
        bond_alb_deinitialize()
            tlb_deinitialize()
		kfree(bond_info->tx_hashtbl)
                bond_info->tx_hashtbl = NULL

Unfortunately if there are still slaves in the bond, when removing the
module the following happens:

    bonding_exit()
        bond_free_all()
            bond_release_all()
                bond_alb_deinit_slave()
                    tlb_clear_slave()
                        tx_hash_table = BOND_ALB_INFO(bond).tx_hashtbl
			u32 next_index = tx_hash_table[index].next

As you might guess we panic when trying to access a few entries into the
table that no longer exists.

I experimented with several options (like moving the calls to
tlb_deinitialize somewhere else), but it really makes the most sense to
be part of the bond_close routine.  It also didn't seem logical move
tlb_clear_slave around too much, so the simplest option seems to add a
check in tlb_clear_slave to make sure we haven't already wiped the
tx_hashtbl away before searching for all the non-existent hash-table
entries that used to point to the slave as the output interface.
Signed-off-by: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>

drivers/net/bonding/bond_alb.c

@@ -169,12 +169,15 @@ static void tlb_clear_slave(struct bonding *bond, struct slave *slave, int save_
 	/* clear slave from tx_hashtbl */
 	tx_hash_table = BOND_ALB_INFO(bond).tx_hashtbl;
 
+	/* skip this if we've already freed the tx hash table */
+	if (tx_hash_table) {
 		index = SLAVE_TLB_INFO(slave).head;
 		while (index != TLB_NULL_INDEX) {
 			u32 next_index = tx_hash_table[index].next;
 			tlb_init_table_entry(&tx_hash_table[index], save_load);
 			index = next_index;
 		}
+	}
 
 	tlb_init_slave(slave);