bonding: Manage /proc/net/bonding/ entries from the netdev events

It was recently reported that moving a bonding device between network
namespaces causes warnings from /proc.  It turns out after the move we
were trying to add and to remove the /proc/net/bonding entries from the
wrong network namespace.

Move the bonding /proc registration code into the NETDEV_REGISTER and
NETDEV_UNREGISTER events where the proc registration and unregistration
will always happen at the right time.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>