btrfs: remove delayed_ref_node from ref_head

This is just excessive information in the ref_head, and makes the code
complicated.  It is a relic from when we had the heads and the refs in
the same tree, which is no longer the case.  With this removal I've
cleaned up a bunch of the cruft around this old assumption as well.

Signed-off-by: Josef Bacik <jbacik@fb.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
index b517ef1..33cba1a 100644
--- a/fs/btrfs/backref.c
+++ b/fs/btrfs/backref.c
@@ -1178,7 +1178,7 @@
 		head = btrfs_find_delayed_ref_head(delayed_refs, bytenr);
 		if (head) {
 			if (!mutex_trylock(&head->mutex)) {
-				refcount_inc(&head->node.refs);
+				refcount_inc(&head->refs);
 				spin_unlock(&delayed_refs->lock);
 
 				btrfs_release_path(path);
@@ -1189,7 +1189,7 @@
 				 */
 				mutex_lock(&head->mutex);
 				mutex_unlock(&head->mutex);
-				btrfs_put_delayed_ref(&head->node);
+				btrfs_put_delayed_ref_head(head);
 				goto again;
 			}
 			spin_unlock(&delayed_refs->lock);