On some practical cases, it is useful to drop new node in the distance.
Because mesh metric is calculated with hop count and without RSSI
information, a node far from local peer and near to destination node
could be used as best path.

For example, the nodes are located in linear. Distance of 0 - 1 and
1 - 2 and 2 - 3 is 20meters. 0 to 3 signal is very weak.

    0 --- 1 --- 2 --- 3

Though most robust path from 0 to 3 is 0 -> 1 -> 2 -> 3,
unfortunately, node 0 could recognize node 3 as neighbor. Then node 3
could be next of node 0. This patch aims to avoid such a case.

[Johannes:]
Dropping the node entirely isn't ideal, but at least with encryption
there will be a limit on # of keys the hardware can deal with, and
there might also be a limit on the number of stations it supports.
Signed-off-by: Masashi Honma <masashi.honma@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>