mac80211: mesh: flush paths outside of plink lock

Lockdep warned of a lock dependency between the mesh_plink lock
and the internal lock for the rhashtable.  The problem is that
the rhashtable code uses a spin lock with softirqs enabled, while
mesh_plink_timer executes a walk (to flush paths on a state change)
inside a softirq with the plink lock held.

This leads to the following deadlock if the timer fires while rht
lock is held on this CPU, and plink lock is held on another CPU:

   CPU0                         CPU1
   ----                         ----
   lock(&(&ht->lock)->rlock);
                                local_irq_disable();
                                lock(&(&sta->mesh->plink_lock)->rlock);
                                lock(&(&ht->lock)->rlock);
   <Interrupt>
   lock(&(&sta->mesh->plink_lock)->rlock);
   *** DEADLOCK ***

Fix by waiting until we drop the plink lock to flush paths.

Fixes: d48a1b7cd439 ("mac80211: mesh: convert path table to rhashtable")
Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>