tipc: enable creating a "preliminary" node

When user sets RX key for a peer not existing on the own node, a new
node entry is needed to which the RX key will be attached. However,
since the peer node address (& capabilities) is unknown at that moment,
only the node-ID is provided, this commit allows the creation of a node
with only the data that we call as “preliminary”.

A preliminary node is not the object of the “tipc_node_find()” but the
“tipc_node_find_by_id()”. Once the first message i.e. LINK_CONFIG comes
from that peer, and is successfully decrypted by the own node, the
actual peer node data will be properly updated and the node will
function as usual.

In addition, the node timer always starts when a node object is created
so if a preliminary node is not used, it will be cleaned up.

The later encryption functions will also use the node timer and be able
to create a preliminary node automatically when needed.

Acked-by: Ying Xue <ying.xue@windreiver.com>
Acked-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: Tuong Lien <tuong.t.lien@dektech.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>