bond: Use active-backup mode on LACP failure.

bond: Use active-backup mode on LACP failure.

Commit bdebeece5 (lacp: Require successful LACP negotiations when
configured.) makes successful LACP negotiation mandatory for the
bond to come UP. This patch provides a configuration option to
bring up the bond by falling back to active-backup mode on LACP
negotiation failure.

Several of the physical switches that support LACP block all traffic
for ports that are configured to use LACP, until LACP is negotiated
with the host. When configuring a LACP bond on a OVS host
(eg: XenServer), this means that there will be an interruption of the
network connectivity between the time the ports on the physical
switch and the bond on the OVS host are configured. The interruption
may be relatively long, if different people are responsible for
managing the switches and the OVS host.

Such network connectivity failure can be avoided if LACP can be
configured on the OVS host before configuring the physical switch,
and having the OVS host fall back to a bond mode (active-backup) till
the physical switch LACP configuration is complete. An option
"lacp-fallback-ab" is introduced with this patch to provide such
behavior on openvswitch.

Signed-off-by: Ravi Kondamuru <Ravi.Kondamuru@citrix.com>
Signed-off-by: Dominic Curran <Dominic.Curran@citrix.com>
Signed-off-by: Ethan Jackson <ethan@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>