bonding: procfs: clean bond->lock usage and use RCU

Use RCU to protect against slave release, the proc show function will sync
with the bond destruction by the proc locks and the fact that the bond is
released after NETDEV_UNREGISTER which causes the bonding to remove the
proc entry.

Signed-off-by: Nikolay Aleksandrov <nikolay@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>