bonding: add __bond_next_slave() which uses neighbours

Add a new function, __bond_next_slave(), which uses neighbours to find the
next slave after the slave provided. It will be further used to gradually
go start using neighbour netdev_adjacent infrastructure instead of
bonding's own lists.

CC: Jay Vosburgh <fubar@us.ibm.com>
CC: Andy Gospodarek <andy@greyhouse.net>
CC: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>