bonding: restructure arp-monitor

In preparation to move the work-queue initialization to port creation
from current port_open phase. Work-queue initialization does not make
sense every time we do 'ifup/ifdown'. So moving to port creation phase.

Arp monitoring work depends on the bonding mode and that is not tied
to the port creation and can change anytime during the life after port
creation. So this restructuring allows us to move the initialization
at creation without losing the ability to arm the correct work for the
mode user has selected.

Signed-off-by: Mahesh Bandewar <maheshb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>