bonding: refactor ARP active-backup monitor

	Refactor ARP monitor for active-backup mode.  The motivation for
this is to take care of locking issues in a clear manner (particularly to
correctly handle RTNL vs. the bonding locks).  Currently, the a-b ARP
monitor does not hold RTNL at all, but future changes will require RTNL
during ARP monitor failovers.

	Rather than using conditional locking, this patch instead breaks
up the ARP monitor into three discrete steps: inspection, commit changes,
and probe.  The inspection phase marks slaves that require link state
changes.  The commit phase is only called if inspection detects that
changes are needed, and is called with RTNL.  Lastly, the probe phase
issues the ARP probes that the inspection phase uses to determine link
state.

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