* Introduced ieee80211_mesh_rt_update that updates a route with the
maximum(lifetime left, new lifetime);
* Modified ieee80211_mesh_route struct by adding a lock that will be used
by both ieee80211_mesh_rt_update and precursor code (added in future commit);
* Modified in ieee80211_hwmp.c HWMP code to use new ieee80211_mesh_rt_update;
* Modified mesh_rt_flush_invalid to use new ieee80211_mesh_rt_update;
* mesh_rt_flush also checks that lifetime == 0, this gives route discovery
a change to complete;
* Modified mesh_recv_mgmt case IEEE80211_FC0_SUBTYPE_BEACON:
when ever we received a beacon from a neighbor we update route lifetime;
Approved by: adrian
