afs: Get rid of the afs_writeback record

Get rid of the afs_writeback record that kAFS is using to match keys with
writes made by that key.

Instead, keep a list of keys that have a file open for writing and/or
sync'ing and iterate through those.

Signed-off-by: David Howells <dhowells@redhat.com>
diff --git a/fs/afs/super.c b/fs/afs/super.c
index af1e769..875b5eb 100644
--- a/fs/afs/super.c
+++ b/fs/afs/super.c
@@ -546,9 +546,9 @@ static void afs_i_init_once(void *_vnode)
 	inode_init_once(&vnode->vfs_inode);
 	mutex_init(&vnode->io_lock);
 	mutex_init(&vnode->validate_lock);
-	spin_lock_init(&vnode->writeback_lock);
+	spin_lock_init(&vnode->wb_lock);
 	spin_lock_init(&vnode->lock);
-	INIT_LIST_HEAD(&vnode->writebacks);
+	INIT_LIST_HEAD(&vnode->wb_keys);
 	INIT_LIST_HEAD(&vnode->pending_locks);
 	INIT_LIST_HEAD(&vnode->granted_locks);
 	INIT_DELAYED_WORK(&vnode->lock_work, afs_lock_work);