LU-7734 lnet: implement Peer Discovery

LU-7734 lnet: implement Peer Discovery

Implement Peer Discovery.

A peer is queued for discovery by lnet_peer_queue_for_discovery().
This sets the LNET_PEER_QUEUED flag (to indicate the lnet_peer is
on the discovery queues) and LNET_PEER_DISCOVERING, to indicate
that discovery is in progress.

The discovery thread lnet_peer_discovery() checks the peer and
updates its state as appropriate.

If LNET_PEER_DATA_PRESENT is set, then a valid Push message or
Ping reply has been received. The peer is updated in accordance
with the data, and LNET_PEER_NIDS_UPTODATE is set.

If LNET_PEER_PING_FAILED is set, then an attempt to send a Ping
message failed, and peer state is updated accordingly. The discovery
thread can do some cleanup like unlinking an MD that cannot be done
from the message event handler.

If LNET_PEER_PUSH_FAILED is set, then an attempt to send a Push
message failed, and peer state is updated accordingly. The discovery
thread can do some cleanup like unlinking an MD that cannot be done
from the message event handler.

If LNET_PEER_PING_REQUIRED is set, we must Ping the peer in order to
correctly update our knowledge of it. This is set, for example, if
we receive a Push message for a peer, but cannot handle it because
the Push target was too small. In such a case we know that the
state of the peer is incorrect, but need to do extra work to obtain
the required information.

If discovery is not enabled, then the discovery process stops here
and the peer is marked with LNET_PEER_UNDISCOVERED. This tells the
discovery process that it doesn't need to revisit the peer while
discovery remains disabled.

If LNET_PEER_NIDS_UPTODATE is not set, then we have reason to think
the lnet_peer is not up to date, and will Ping it.

The peer needs a Push if it is multi-rail and the ping buffer
sequence number for this node is newer than the sequence number it
has acknowledged receiving by sending an Ack of a Push.

If none of the above is true, then discovery has completed its work
on the peer.

Discovery signals that it is done with a peer by clearing the
LNET_PEER_DISCOVERING flag, and setting LNET_PEER_DISCOVERED or
LNET_PEER_UNDISCOVERED as appropriate. It then dequeues the peer
and clears the LNET_PEER_QUEUED flag.

When the local node is discovered via the loopback network, the
peer structure that is created will have an lnet_peer_ni for the
local loopback interface. Subsequent traffic from this node to
itself will use the loopback net.

Test-Parameters: trivial
Signed-off-by: Olaf Weber <olaf@sgi.com>
Change-Id: I9fbc0528788c352580c6d0449c7c66cba2740df2