Btrfs: use tokens where we can in the tree log

If we are syncing over and over the overhead of doing all those maps in
fill_inode_item and log_changed_extents really starts to hurt, so use map
tokens so we can avoid all the extra mapping.  Since the token maps from our
offset to the end of the page make sure to set the first thing in the item
first so we really only do one map.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>