btrfs: fix the commment for the action flags in delayed-ref.h

The action field has been merged into struct btrfs_delayed_ref_node,
and no struct btrfs_delayed_ref is available now.

Signed-off-by: Wang Sheng-Hui <shhuiw@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>