mac80211: mesh: drop new node with weak power

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>