LCP IP allocation algorithm - new function lcp2_failback_candidate()

LCP IP allocation algorithm - new function lcp2_failback_candidate()

There's a bug in LCP2.  Selecting the node with the highest imbalance
doesn't always work.  Some nodes can have a high imbalance metric
because they have a lot of IPs.  However, these nodes can be part of a
group that is perfectly balanced.  Nodes in another group with less
IPs might actually be imbalanced.

Factor out the code from lcp2_failback() that actually takes a node
and decides which address should be moved to which node.

This is the first step in fixing the above bug.

Signed-off-by: Martin Schwenke <martin@meltin.net>