bonding: Clean up resource leaks

This patch reworks the resource free logic performed at the time
a bonding device is released.  This (a) closes two resource leaks, one
for workqueues and one for multicast lists, and (b) improves commonality
of code between the "destroy one" and "destroy all" paths by performing
final free activity via destructor instead of explicitly (and differently)
in each path.

"Sean E. Millichamp" <sean@bruenor.org> reported the workqueue
leak, and included a different patch.

Signed-off-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>