[BONDING]: Fix resource use after free

Fix bond_destroy and bond_free_all to not reference the struct
net_device after calling unregister_netdevice.

Bug and offending change reported by Moni Shoua <monis@voltaire.com>

Signed-off-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>