bonding: fix set_multicast_list locking

	This patch eliminates a problem (reported by lockdep) in the
bond_set_multicast_list function.  It first reduces the locking on
bond->lock to a simple read_lock, and second, adds netif_tx locking
around the bonding mc_list manipulations that occur outside of the
set_multicast_list function.

	The original problem was related to IPv6 addrconf activity.

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