Merge branch 'protodown'

Anuradha Karuppiah says:

====================
net: Introduce protodown flag.

User space daemons can detect errors in the network that need to be
notified to the switch device drivers.

Drivers can react to this error state by doing a phy-down on the
switch-port which would result in a carrier-off locally and on the directly
connected switch. Doing that would prevent loops and black-holes in the
network.

One such use case is the multi-chassis LAG application -

1. The MLAG application runs on peer switches (say Switch0 and Switch1)
   synchronizing states, forwarding entries etc. between the two
   switches over the peer-link (this is a link directly connecting the
   two switches).
2. An MLAG election process designates one of the switches as a primary
   (for e.g. Switch0 is primary and Switch1 is secondary).
3. The peer link plays a critical role in allowing Switch0-Switch1 to
   function as a single LAG partner to the downstream dual-connected
   servers. When the peer-link between the switches goes down we have a
   split-brain situation. Switch0 and Switch1 are no longer in sync and
   are acting independently. This can result in traffic loops and
   traffic black-holing in the network.
4. To prevent these problems the MLAG application on the secondary
   switch phy-downs the MLAG ports on detecting the peer-link down.
   This will be seen as a carrier down on servers that are
   dual-connected to Switch0 and Switch1.
5. Specifically a dual-connected server will see a carrier-down on the
   port connected to the MLAG secondary, Switch1, and will stop using
   that port for traffic TX. So traffic black holing is prevented.

v6 to v7:
   Removed some unnecessary code in response to review comments.

v5 to v6:
   Replaced proto_flags with a simple proto_down boolean attribute in
   response to Dave's comments.

v4 to v5:
   Changed the ip link display format for protodown to match the set as
   recommended by Stephen.

v3 to v4:
   I have moved protodown out of IFF_XXX and introduced a separate
   proto_flags field with IF_PROTOF_DOWN bit being used by apps to notify
   switch port errors. This is in response to Stephen's comments that
   adding a new IFF_XXX may break user space.

   I have used rocker as the sample switch driver. And to test this
   functionality I used the qemu-rocker patch that Scott sent out in
   response to the v3 posting (needed to set link up/down when phy is
   enabled/disabled).

v1 to v2:
   Based on Dave's suggestion I have moved out aggregating of error bits
   across applications to a user space framework. This patch now simply
   notifies an aggregated error bit to drivers enabling them to handle
   the error gracefully.
====================

Signed-off-by: David S. Miller <davem@davemloft.net>