afs: Overhaul cell database management

Overhaul the way that the in-kernel AFS client keeps track of cells in the
following manner:

 (1) Cells are now held in an rbtree to make walking them quicker and RCU
     managed (though this is probably overkill).

 (2) Cells now have a manager work item that:

     (A) Looks after fetching and refreshing the VL server list.

     (B) Manages cell record lifetime, including initialising and
     	 destruction.

     (B) Manages cell record caching whereby threads are kept around for a
     	 certain time after last use and then destroyed.

     (C) Manages the FS-Cache index cookie for a cell.  It is not permitted
     	 for a cookie to be in use twice, so we have to be careful to not
     	 allow a new cell record to exist at the same time as an old record
     	 of the same name.

 (3) Each AFS network namespace is given a manager work item that manages
     the cells within it, maintaining a single timer to prod cells into
     updating their DNS records.

     This uses the reduce_timer() facility to make the timer expire at the
     soonest timed event that needs happening.

 (4) When a module is being unloaded, cells and cell managers are now
     counted out using dec_after_work() to make sure the module text is
     pinned until after the data structures have been cleaned up.

 (5) Each cell's VL server list is now protected by a seqlock rather than a
     semaphore.

Signed-off-by: David Howells <dhowells@redhat.com>

fs/afs/main.c

@@ -46,12 +46,15 @@ static int __net_init afs_net_init(struct afs_net *net)
 
 	INIT_WORK(&net->charge_preallocation_work, afs_charge_preallocation);
 	mutex_init(&net->socket_mutex);
-	INIT_LIST_HEAD(&net->cells);
-	rwlock_init(&net->cells_lock);
-	init_rwsem(&net->cells_sem);
-	init_waitqueue_head(&net->cells_freeable_wq);
-	init_rwsem(&net->proc_cells_sem);
+
+	net->cells = RB_ROOT;
+	seqlock_init(&net->cells_lock);
+	INIT_WORK(&net->cells_manager, afs_manage_cells);
+	timer_setup(&net->cells_timer, afs_cells_timer, 0);
+
+	spin_lock_init(&net->proc_cells_lock);
 	INIT_LIST_HEAD(&net->proc_cells);
+
 	INIT_LIST_HEAD(&net->vl_updates);
 	INIT_LIST_HEAD(&net->vl_graveyard);
 	INIT_DELAYED_WORK(&net->vl_reaper, afs_vlocation_reaper);
@@ -83,11 +86,14 @@ static int __net_init afs_net_init(struct afs_net *net)
 	return 0;
 
 error_open_socket:
+	net->live = false;
 	afs_vlocation_purge(net);
 	afs_cell_purge(net);
 error_cell_init:
+	net->live = false;
 	afs_proc_cleanup(net);
 error_proc:
+	net->live = false;
 	return ret;
 }