bonding: make bond_for_each_slave() use lower neighbour's private

It needs a list_head *iter, so add it wherever needed. Use both non-rcu and
rcu variants.

CC: Jay Vosburgh <fubar@us.ibm.com>
CC: Andy Gospodarek <andy@greyhouse.net>
CC: Dimitris Michailidis <dm@chelsio.com>
Signed-off-by: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>