btrfs: clear defragmented inodes using postorder in btrfs_cleanup_defrag_inodes()

btrfs_cleanup_defrag_inodes() is not called frequently, only in remount
or unmount, but the way it frees the inodes in fs_info->defrag_inodes
is inefficient. Each time it needs to locate first node, remove it,
potentially rebalance tree until it's done. This allows to do a
conditional reschedule.

For cleanups the rbtree_postorder_for_each_entry_safe() iterator is
convenient but we can't reschedule and restart iteration because some of
the tree nodes would be already freed.

The cleanup operation is kmem_cache_free() which will likely take the
fast path for most objects so rescheduling should not be necessary.
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/defrag.c

@@ -212,20 +212,14 @@ static struct inode_defrag *btrfs_pick_defrag_inode(
 
 void btrfs_cleanup_defrag_inodes(struct btrfs_fs_info *fs_info)
 {
-	struct inode_defrag *defrag;
-	struct rb_node *node;
+	struct inode_defrag *defrag, *next;
 
 	spin_lock(&fs_info->defrag_inodes_lock);
-	node = rb_first(&fs_info->defrag_inodes);
-	while (node) {
-		rb_erase(node, &fs_info->defrag_inodes);
-		defrag = rb_entry(node, struct inode_defrag, rb_node);
-		kmem_cache_free(btrfs_inode_defrag_cachep, defrag);
 
-		cond_resched_lock(&fs_info->defrag_inodes_lock);
+	rbtree_postorder_for_each_entry_safe(defrag, next,
+					     &fs_info->defrag_inodes, rb_node)
+		kmem_cache_free(btrfs_inode_defrag_cachep, defrag);
 
-		node = rb_first(&fs_info->defrag_inodes);
-	}
 	spin_unlock(&fs_info->defrag_inodes_lock);
 }